Plots don't show automatically when PyPlot compiled with PackageCompiler

Not sure if this is supposed to work (in any case it seems like its really close to working). I compiled PyPlot v2.8.2 with PackageCompiler v1.1.0 on julia 1.4,

```julia
create_sysimage([:PyPlot], sysimage_path="pyplot.so")
```

then load it with

```julia
$ julia -J pyplot.so

julia> using PyPlot

# no plot gets shown shown here (but there should be)
julia> plot([1,2,3])
1-element Array{PyCall.PyObject,1}:
PyObject

# this shows the plot, but the following error pops up after closing the window
julia> show()
ERROR: PyError ($(Expr(:escape, :(ccall(#= /home/marius/.julia/packages/PyCall/zqDXB/src/pyfncall.jl:43 =# @pysym(:PyObject_Call), PyPtr, (PyPtr, PyPtr, PyPtr), o, pyargsptr, kw)))))
TypeError('signal handler must be signal.SIG_IGN, signal.SIG_DFL, or a callable object')
File "/home/marius/miniconda3/lib/python3.7/site-packages/matplotlib/pyplot.py", line 269, in show
return _show(*args, **kw)
File "/home/marius/miniconda3/lib/python3.7/site-packages/matplotlib/cbook/deprecation.py", line 413, in wrapper
return func(*args, **kwargs)
File "/home/marius/miniconda3/lib/python3.7/site-packages/matplotlib/backend_bases.py", line 3302, in show
cls.mainloop()
File "/home/marius/miniconda3/lib/python3.7/site-packages/matplotlib/backends/backend_qt5.py", line 1099, in mainloop
signal.signal(signal.SIGINT, old_signal)
File "/home/marius/miniconda3/lib/python3.7/signal.py", line 47, in signal
handler = _signal.signal(_enum_to_int(signalnum), _enum_to_int(handler))

Stacktrace:
[1] pyerr_check at /home/marius/.julia/packages/PyCall/zqDXB/src/exception.jl:60 [inlined]
[2] pyerr_check at /home/marius/.julia/packages/PyCall/zqDXB/src/exception.jl:64 [inlined]
[3] _handle_error(::String) at /home/marius/.julia/packages/PyCall/zqDXB/src/exception.jl:81
[4] macro expansion at /home/marius/.julia/packages/PyCall/zqDXB/src/exception.jl:95 [inlined]
[5] #110 at /home/marius/.julia/packages/PyCall/zqDXB/src/pyfncall.jl:43 [inlined]
[6] disable_sigint at ./c.jl:446 [inlined]
[7] __pycall! at /home/marius/.julia/packages/PyCall/zqDXB/src/pyfncall.jl:42 [inlined]
[8] _pycall!(::PyCall.PyObject, ::PyCall.PyObject, ::Tuple{}, ::Int64, ::Ptr{Nothing}) at /home/marius/.julia/packages/PyCall/zqDXB/src/pyfncall.jl:29
[9] _pycall! at /home/marius/.julia/packages/PyCall/zqDXB/src/pyfncall.jl:11 [inlined]
[10] #pycall#115 at /home/marius/.julia/packages/PyCall/zqDXB/src/pyfncall.jl:80 [inlined]
[11] pycall at /home/marius/.julia/packages/PyCall/zqDXB/src/pyfncall.jl:80 [inlined]
[12] #show#148 at /home/marius/.julia/packages/PyPlot/4wzW1/src/PyPlot.jl:183 [inlined]
[13] show() at /home/marius/.julia/packages/PyPlot/4wzW1/src/PyPlot.jl:183
[14] top-level scope at REPL[3]:1
```

Any ideas if this can work and if so how? Thanks.

Reproduce the issue with the shown PackageCompiler sysimage command and Julia REPL steps. Start at src/PyPlot.jl:183 and compare the PyCall traceback around show(); done means plots display automatically after using PyPlot and show() closes without the reported signal-handler error.

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