pyjlwrap support for Pickle Serialization

PyCall works nicely for many use cases between Python and Julia. In particular, there is one that could be improved and very important for Data Scientist community. For example, I tried to use it for PySpark library and works very well for the basic use case. But, if the user needs to create a UDF (User Defined Functions), the user will have trouble to serialize the functions.
The UDFs, in this case, would help to many DSs reuse Julia code and call spark to do the heavy work. Have this enabled, would improve the usage of Julia in different scenarios.

To solve the current issues with UDF, PyObject needs to be serializable with Pickle. I don't have much idea how to solve this, but I have a simple use case that if we fix would improve towards this functionality:

**Example:**
```
using PyCall
pickle = pyimport("pickle")
pickle.dumps(x -> x + 1)
```

**Error:**
```
ERROR: PyError ($(Expr(:escape, :(ccall(#= /root/.julia/packages/PyCall/zqDXB/src/pyfncall.jl:43 =# @pysym(:PyObject_Call), PyPtr, (PyPtr, PyPtr, PyPtr), o, pyargsptr, kw)))))
TypeError("cannot pickle 'PyCall.jlwrap' object")

Stacktrace:
[1] pyerr_check at /root/.julia/packages/PyCall/zqDXB/src/exception.jl:60 [inlined]
[2] pyerr_check at /root/.julia/packages/PyCall/zqDXB/src/exception.jl:64 [inlined]
[3] _handle_error(::String) at /root/.julia/packages/PyCall/zqDXB/src/exception.jl:81
[4] macro expansion at /root/.julia/packages/PyCall/zqDXB/src/exception.jl:95 [inlined]
[5] #110 at /root/.julia/packages/PyCall/zqDXB/src/pyfncall.jl:43 [inlined]
[6] disable_sigint at ./c.jl:446 [inlined]
[7] __pycall! at /root/.julia/packages/PyCall/zqDXB/src/pyfncall.jl:42 [inlined]
[8] _pycall!(::PyObject, ::PyObject, ::Tuple{var"#3#4"}, ::Int64, ::Ptr{Nothing}) at /root/.julia/packages/PyCall/zqDXB/src/pyfncall.jl:29
[9] _pycall!(::PyObject, ::PyObject, ::Tuple{var"#3#4"}, ::Base.Iterators.Pairs{Union{},Union{},Tuple{},NamedTuple{(),Tuple{}}}) at /root/.julia/packages/PyCall/zqDXB/src/pyfncall.jl:11
[10] (::PyObject)(::Function; kwargs::Base.Iterators.Pairs{Union{},Union{},Tuple{},NamedTuple{(),Tuple{}}}) at /root/.julia/packages/PyCall/zqDXB/src/pyfncall.jl:86
[11] (::PyObject)(::Function) at /root/.julia/packages/PyCall/zqDXB/src/pyfncall.jl:86
[12] top-level scope at REPL[13]:1
```

Reference to UDF in Python: https://docs.databricks.com/spark/latest/spark-sql/udf-python.html

Reproduce the pickle.dumps example from the issue and start by reading the PyObject handling and the pyfncall.jl path shown in the traceback. Done means PyObject-backed functions can be serialized for the described UDF use case, with the reported failure covered by a regression test.

Vom Indexierungsmodell aus dem Issue-Text verfasst.