pyjulia segfaults with multiprocessing

Calling Julia code using Python's multiprocessing module crashes the program (SIGSEGV). I am using Julia 1.2.0, Python 3.7.3 and pyjulia 0.4.1 on Linux.

A minimal example is as follows:
```
import multiprocessing as mp
import julia

def test(*args):
j = julia.Julia()

myfunc = j.eval("function test() return 5; end")
myfunc()

with mp.Pool(10) as p:
p.map(test, range(10))
```

This program crashes with a high probability which increases with the number of children. Based on the actual code I need to run it seems like the crash occurs whenever a Julia-defined function is called for the first time in a process; if this succeeds the program does not seem to crash even after many iterations.

```
Warning: `getindex(o::PyObject, s::Symbol)` is deprecated in favor of dot overloading (`getproperty`) so elements should now be accessed as e.g. `o.s` instead of `o[:s]`.
│ caller = top-level scope at none:4
└ @ Core none:4

signal (11): Segmentation fault
in expression starting at none:1
unknown function (ip: 0xffffffffffffffff)
jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2197
jl_apply at /buildworker/worker/package_linux64/build/src/julia.h:1614 [inlined]
call_require at /buildworker/worker/package_linux64/build/src/toplevel.c:399 [inlined]
eval_import_path at /buildworker/worker/package_linux64/build/src/toplevel.c:436
jl_toplevel_eval_flex at /buildworker/worker/package_linux64/build/src/toplevel.c:656
jl_toplevel_eval_flex at /buildworker/worker/package_linux64/build/src/toplevel.c:764
jl_toplevel_eval_in at /buildworker/worker/package_linux64/build/src/toplevel.c:844
jl_eval_string at /buildworker/worker/package_linux64/build/src/jlapi.c:94
ffi_call_unix64 at /home/user/.miniconda3/lib/python3.7/lib-dynload/../../libffi.so.6 (unknown line)
ffi_call at /home/user/.miniconda3/lib/python3.7/lib-dynload/../../libffi.so.6 (unknown line)
_call_function_pointer at /usr/local/src/conda/python-3.7.3/Modules/_ctypes/callproc.c:827 [inlined]
_ctypes_callproc at /usr/local/src/conda/python-3.7.3/Modules/_ctypes/callproc.c:1184
PyCFuncPtr_call at /usr/local/src/conda/python-3.7.3/Modules/_ctypes/_ctypes.c:3969
_PyObject_FastCallKeywords at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
call_function.lto_priv.1536 at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyEval_EvalFrameDefault at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
function_code_fastcall at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
call_function.lto_priv.1536 at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyEval_EvalFrameDefault at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyEval_EvalCodeWithName at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyFunction_FastCallDict at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyObject_Call_Prepend at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
slot_tp_init at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
type_call.lto_priv.151 at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
PyObject_Call at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyEval_EvalFrameDefault at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyEval_EvalCodeWithName at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyFunction_FastCallDict at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyObject_Call_Prepend at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
slot_tp_init at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
type_call.lto_priv.151 at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyObject_FastCallKeywords at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
call_function.lto_priv.1536 at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyEval_EvalFrameDefault at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyEval_EvalCodeWithName at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyFunction_FastCallDict at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
map_next.lto_priv.527 at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
list_extend.lto_priv.888 at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
list___init__.lto_priv.379 at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
type_call.lto_priv.151 at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyObject_FastCallKeywords at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
call_function.lto_priv.1536 at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyEval_EvalFrameDefault at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
function_code_fastcall at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyFunction_FastCallDict at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyEval_EvalFrameDefault at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyEval_EvalCodeWithName at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyFunction_FastCallDict at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyEval_EvalFrameDefault at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
function_code_fastcall at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
call_function.lto_priv.1536 at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyEval_EvalFrameDefault at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
function_code_fastcall at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
call_function.lto_priv.1536 at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyEval_EvalFrameDefault at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
function_code_fastcall at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
call_function.lto_priv.1536 at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyEval_EvalFrameDefault at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
function_code_fastcall at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyFunction_FastCallDict at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyObject_Call_Prepend at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
slot_tp_init at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
type_call.lto_priv.151 at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyObject_FastCallKeywords at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
call_function.lto_priv.1536 at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyEval_EvalFrameDefault at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
function_code_fastcall at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
call_function.lto_priv.1536 at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyEval_EvalFrameDefault at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
function_code_fastcall at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
call_function.lto_priv.1536 at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyEval_EvalFrameDefault at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
function_code_fastcall at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
call_function.lto_priv.1536 at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyEval_EvalFrameDefault at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyEval_EvalCodeWithName at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyFunction_FastCallDict at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyObject_Call_Prepend at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
slot_tp_init at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
type_call.lto_priv.151 at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyObject_FastCallKeywords at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
call_function.lto_priv.1536 at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyEval_EvalFrameDefault at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyEval_EvalCodeWithName at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyFunction_FastCallKeywords at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
call_function.lto_priv.1536 at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyEval_EvalFrameDefault at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyEval_EvalCodeWithName at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
PyEval_EvalCodeEx at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
PyEval_EvalCode at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
builtin_exec at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyMethodDef_RawFastCallKeywords at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyCFunction_FastCallKeywords at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
call_function.lto_priv.1536 at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyEval_EvalFrameDefault at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyEval_EvalCodeWithName at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyFunction_FastCallKeywords at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
call_function.lto_priv.1536 at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyEval_EvalFrameDefault at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyEval_EvalCodeWithName at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyFunction_FastCallKeywords at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
call_function.lto_priv.1536 at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyEval_EvalFrameDefault at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyEval_EvalCodeWithName at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyFunction_FastCallKeywords at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
call_function.lto_priv.1536 at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyEval_EvalFrameDefault at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyEval_EvalCodeWithName at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyFunction_FastCallDict at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyEval_EvalFrameDefault at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyEval_EvalCodeWithName at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
_PyFunction_FastCallDict at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
macro expansion at /home/user/.julia/packages/PyCall/ttONZ/src/exception.jl:81 [inlined]
__pycall! at /home/user/.julia/packages/PyCall/ttONZ/src/pyfncall.jl:44
_pycall! at /home/user/.julia/packages/PyCall/ttONZ/src/pyfncall.jl:29
#call#111 at /home/user/.julia/packages/PyCall/ttONZ/src/pyfncall.jl:11 [inlined]
PyObject at /home/user/.julia/packages/PyCall/ttONZ/src/pyfncall.jl:89
jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2197
do_call at /buildworker/worker/package_linux64/build/src/interpreter.c:323
eval_value at /buildworker/worker/package_linux64/build/src/interpreter.c:411
eval_body at /buildworker/worker/package_linux64/build/src/interpreter.c:635
jl_interpret_toplevel_thunk_callback at /buildworker/worker/package_linux64/build/src/interpreter.c:884
unknown function (ip: 0xfffffffffffffffe)
unknown function (ip: 0x7f67cc984f0f)
unknown function (ip: 0x2)
jl_interpret_toplevel_thunk at /buildworker/worker/package_linux64/build/src/interpreter.c:893
jl_toplevel_eval_flex at /buildworker/worker/package_linux64/build/src/toplevel.c:815
jl_toplevel_eval_flex at /buildworker/worker/package_linux64/build/src/toplevel.c:764
jl_toplevel_eval_in at /buildworker/worker/package_linux64/build/src/toplevel.c:844
eval at ./boot.jl:330
jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2191
exec_options at ./client.jl:271
_start at ./client.jl:464
jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2191
unknown function (ip: 0x40192d)
unknown function (ip: 0x401533)
__libc_start_main at /lib64/libc.so.6 (unknown line)
unknown function (ip: 0x4015d4)
Allocations: 11458870 (Pool: 11456484; Big: 2386); GC: 25
^C^C^C^C^C^C^CWARNING: Force throwing a SIGINT

signal (11): Segmentation fault
in expression starting at none:4
tupledealloc.lto_priv.484 at /home/user/.miniconda3/lib/libpython3.7m.so.1.0 (unknown line)
pydecref_ at /home/user/.julia/packages/PyCall/ttONZ/src/PyCall.jl:121
_pycall! at /home/user/.julia/packages/PyCall/ttONZ/src/pyfncall.jl:31
#call#111 at /home/user/.julia/packages/PyCall/ttONZ/src/pyfncall.jl:11 [inlined]
PyObject at /home/user/.julia/packages/PyCall/ttONZ/src/pyfncall.jl:89
jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2197
do_call at /buildworker/worker/package_linux64/build/src/interpreter.c:323
eval_value at /buildworker/worker/package_linux64/build/src/interpreter.c:411
eval_body at /buildworker/worker/package_linux64/build/src/interpreter.c:635
jl_interpret_toplevel_thunk_callback at /buildworker/worker/package_linux64/build/src/interpreter.c:884
unknown function (ip: 0xfffffffffffffffe)
unknown function (ip: 0x7f67cc984f0f)
unknown function (ip: 0x2)
jl_interpret_toplevel_thunk at /buildworker/worker/package_linux64/build/src/interpreter.c:893
jl_toplevel_eval_flex at /buildworker/worker/package_linux64/build/src/toplevel.c:815
jl_toplevel_eval_flex at /buildworker/worker/package_linux64/build/src/toplevel.c:764
jl_toplevel_eval_in at /buildworker/worker/package_linux64/build/src/toplevel.c:844
^Ceval at ./boot.jl:330
jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2191
exec_options at ./client.jl:271
_start at ./client.jl:464
jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2191
unknown function (ip: 0x40192d)^C
unknown function (ip: 0x401533)
__libc_start_main at /lib64/libc.so.6 (unknown line)
unknown function (ip: 0x4015d4)
Allocations: 8918253 (Pool: 8916828; Big: 1425); GC: 19
/bin/bash: line 1: 24040 Segmentation fault
```

This seems related to #344 and #306 (where I recently remarked the issue). Using `python-jl` does not seem to make a difference.