Segmentation fault when registering Julia-defined Python class with Matplotlib

I have implemented a custom axis transform [according to the documentation](https://matplotlib.org/stable/api/transformations.html):

```julia
SqrtTransform = pytype("SqrtTransform", (matplotlib.transforms.Transform,), [
"__module__" => "__main__",
pyfunc(name="__init__",
function (self)
self.input_dims = 1
self.output_dims = 1
self.is_separable = true
self.has_inverse = true
matplotlib.transforms.Transform.__init__(self)
end),
pyfunc(name="transform_non_affine",
function(self, a)
b = pyconvert(Array, a)
v = similar(b)
sel = b .>= 0
v[sel] .= .√(b[sel])
# v[.!sel] .= NaN
v
end),
pyfunc(name="inverted",
(self) -> SquareTransform())
])

SquareTransform = pytype("SquareTransform", (matplotlib.transforms.Transform,), [
"__module__" => "__main__",
pyfunc(name="__init__",
function (self)
self.input_dims = 1
self.output_dims = 1
self.is_separable = true
self.has_inverse = true
matplotlib.transforms.Transform.__init__(self)
end),
pyfunc(name="transform_non_affine",
function(self, a)
pyconvert(Array, a) .^ 2
end),
pyfunc(name="inverted",
(self) -> SqrtTransform())
])

SqrtScale = pytype("SqrtScale", (matplotlib.scale.ScaleBase,), [
"__module__" => "__main__",
"name" => "sqrt",
pyfunc(name="__init__",
function(self, axis, args...; kwargs...)
matplotlib.scale.ScaleBase.__init__(self, axis)
end),
pyfunc(name="get_transform",
(self) -> SqrtTransform()),
pyfunc(name="set_default_locators_and_formatters",
(self, axis) -> nothing),
pyfunc(name="limit_range_for_scale",
function(self, vmin, vmax, minpos)
max(vmin, 0), max(vmax, 0)
end)])
```

To register this with Matplotlib, one uses
```julia
matplotlib.scale.register_scale(SqrtScale)
```

This works well from a script, but not if I try to put it into a package. If I just put the line registering the transform with Matplotlib after the class definitions, nothing happens, i.e. Matplotlib does not in fact register the new transform. If I instead try to register it from `__init__()`, or from the REPL after having loaded the package, it crashes with the following trace:

```julia
signal (11): Segmentation fault: 11
in expression starting at REPL[4]:1
_PyFunction_Vectorcall at /Users/jagot/work/projects/move-to-pythoncall/.CondaPkg/env/lib/libpython3.11.dylib (unknown line)
PyObject_CallObject at /Users/jagot/.julia/packages/PythonCall/dsECZ/src/cpython/pointers.jl:299 [inlined]
pycallargs at /Users/jagot/.julia/packages/PythonCall/dsECZ/src/abstract/object.jl:210
unknown function (ip: 0x10ce5a8e6)
ijl_apply_generic at /Users/jagot/.julia/juliaup/julia-1.8.5+0.x64.apple.darwin14/lib/julia/libjulia-internal.1.8.dylib (unknown line)
#pycall#59 at /Users/jagot/.julia/packages/PythonCall/dsECZ/src/abstract/object.jl:228
ijl_apply_generic at /Users/jagot/.julia/juliaup/julia-1.8.5+0.x64.apple.darwin14/lib/julia/libjulia-internal.1.8.dylib (unknown line)
do_apply at /Users/jagot/.julia/juliaup/julia-1.8.5+0.x64.apple.darwin14/lib/julia/libjulia-internal.1.8.dylib (unknown line)
pycall at /Users/jagot/.julia/packages/PythonCall/dsECZ/src/abstract/object.jl:218
ijl_apply_generic at /Users/jagot/.julia/juliaup/julia-1.8.5+0.x64.apple.darwin14/lib/julia/libjulia-internal.1.8.dylib (unknown line)
do_apply at /Users/jagot/.julia/juliaup/julia-1.8.5+0.x64.apple.darwin14/lib/julia/libjulia-internal.1.8.dylib (unknown line)
#_#11 at /Users/jagot/.julia/packages/PythonCall/dsECZ/src/Py.jl:352
ijl_apply_generic at /Users/jagot/.julia/juliaup/julia-1.8.5+0.x64.apple.darwin14/lib/julia/libjulia-internal.1.8.dylib (unknown line)
do_apply at /Users/jagot/.julia/juliaup/julia-1.8.5+0.x64.apple.darwin14/lib/julia/libjulia-internal.1.8.dylib (unknown line)
Py at /Users/jagot/.julia/packages/PythonCall/dsECZ/src/Py.jl:352
ijl_apply_generic at /Users/jagot/.julia/juliaup/julia-1.8.5+0.x64.apple.darwin14/lib/julia/libjulia-internal.1.8.dylib (unknown line)
do_call at /Users/jagot/.julia/juliaup/julia-1.8.5+0.x64.apple.darwin14/lib/julia/libjulia-internal.1.8.dylib (unknown line)
eval_body at /Users/jagot/.julia/juliaup/julia-1.8.5+0.x64.apple.darwin14/lib/julia/libjulia-internal.1.8.dylib (unknown line)
jl_interpret_toplevel_thunk at /Users/jagot/.julia/juliaup/julia-1.8.5+0.x64.apple.darwin14/lib/julia/libjulia-internal.1.8.dylib (unknown line)
jl_toplevel_eval_flex at /Users/jagot/.julia/juliaup/julia-1.8.5+0.x64.apple.darwin14/lib/julia/libjulia-internal.1.8.dylib (unknown line)
jl_toplevel_eval_flex at /Users/jagot/.julia/juliaup/julia-1.8.5+0.x64.apple.darwin14/lib/julia/libjulia-internal.1.8.dylib (unknown line)
jl_toplevel_eval_flex at /Users/jagot/.julia/juliaup/julia-1.8.5+0.x64.apple.darwin14/lib/julia/libjulia-internal.1.8.dylib (unknown line)
ijl_toplevel_eval_in at /Users/jagot/.julia/juliaup/julia-1.8.5+0.x64.apple.darwin14/lib/julia/libjulia-internal.1.8.dylib (unknown line)
eval at ./boot.jl:368 [inlined]
eval_user_input at /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-macmini-x64-5.0/build/default-macmini-x64-5-0/julialang/julia-release-1-dot-8/usr/share/julia/stdlib/v1.8/REPL/src/REPL.jl:151
repl_backend_loop at /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-macmini-x64-5.0/build/default-macmini-x64-5-0/julialang/julia-release-1-dot-8/usr/share/julia/stdlib/v1.8/REPL/src/REPL.jl:247
start_repl_backend at /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-macmini-x64-5.0/build/default-macmini-x64-5-0/julialang/julia-release-1-dot-8/usr/share/julia/stdlib/v1.8/REPL/src/REPL.jl:232
#run_repl#47 at /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-macmini-x64-5.0/build/default-macmini-x64-5-0/julialang/julia-release-1-dot-8/usr/share/julia/stdlib/v1.8/REPL/src/REPL.jl:369
run_repl at /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-macmini-x64-5.0/build/default-macmini-x64-5-0/julialang/julia-release-1-dot-8/usr/share/julia/stdlib/v1.8/REPL/src/REPL.jl:355
jfptr_run_repl_63566.clone_1 at /Users/jagot/.julia/juliaup/julia-1.8.5+0.x64.apple.darwin14/lib/julia/sys.dylib (unknown line)
ijl_apply_generic at /Users/jagot/.julia/juliaup/julia-1.8.5+0.x64.apple.darwin14/lib/julia/libjulia-internal.1.8.dylib (unknown line)
#967 at ./client.jl:419
jfptr_YY.967_56999.clone_1 at /Users/jagot/.julia/juliaup/julia-1.8.5+0.x64.apple.darwin14/lib/julia/sys.dylib (unknown line)
ijl_apply_generic at /Users/jagot/.julia/juliaup/julia-1.8.5+0.x64.apple.darwin14/lib/julia/libjulia-internal.1.8.dylib (unknown line)
jl_f__call_latest at /Users/jagot/.julia/juliaup/julia-1.8.5+0.x64.apple.darwin14/lib/julia/libjulia-internal.1.8.dylib (unknown line)
#invokelatest#2 at ./essentials.jl:729 [inlined]
invokelatest at ./essentials.jl:726 [inlined]
run_main_repl at ./client.jl:404
exec_options at ./client.jl:318
_start at ./client.jl:522
jfptr__start_57423.clone_1 at /Users/jagot/.julia/juliaup/julia-1.8.5+0.x64.apple.darwin14/lib/julia/sys.dylib (unknown line)
ijl_apply_generic at /Users/jagot/.julia/juliaup/julia-1.8.5+0.x64.apple.darwin14/lib/julia/libjulia-internal.1.8.dylib (unknown line)
true_main at /Users/jagot/.julia/juliaup/julia-1.8.5+0.x64.apple.darwin14/lib/julia/libjulia-internal.1.8.dylib (unknown line)
jl_repl_entrypoint at /Users/jagot/.julia/juliaup/julia-1.8.5+0.x64.apple.darwin14/lib/julia/libjulia-internal.1.8.dylib (unknown line)
Allocations: 19584391 (Pool: 19573667; Big: 10724); GC: 22
```

Environment:
```julia
Status `~/work/projects/move-to-pythoncall/Project.toml`
[84fb645a] Jagot v0.1.0 `~/.julia/dev/Jagot`
[6099a3de] PythonCall v0.9.12
[274fc56d] PythonPlot v1.0.2
[295af30f] Revise v3.5.2
```