EXCEPTION_ACCESS_VIOLATION on Windows x86 with Julia nightly since Julia 1.2.0-DEV.641

Tests on Windows x86 with Julia nightly have been failing for all Python versions since (at least) Julia 1.2.0-DEV.641 ([logs](https://ci.appveyor.com/project/StevenGJohnson/pycall-jl-nu3aa/builds/23588267)). This still happened in the latest tests on PyCall master with Julia 1.3.0-DEV.2 ([logs](https://ci.appveyor.com/project/StevenGJohnson/pycall-jl-nu3aa/builds/23785003)). The last success was with Julia 1.2.0-DEV.570 ([logs](https://ci.appveyor.com/project/StevenGJohnson/pycall-jl-nu3aa/builds/23463508)).

The error happens in `__pycall!`:

```
Exception: EXCEPTION_ACCESS_VIOLATION at 0xff60240 -- unknown function (ip: 0FF60240)
in expression starting at C:\projects\pycall-jl-nu3aa\test\runtests.jl:29
unknown function (ip: 0FF6023F)
PyObject_Call at C:\Python37\python37.dll (unknown line)
macro expansion at C:\projects\pycall-jl-nu3aa\src\exception.jl:81 [inlined]
__pycall! at C:\projects\pycall-jl-nu3aa\src\pyfncall.jl:44
_pycall! at C:\projects\pycall-jl-nu3aa\src\pyfncall.jl:29
_pycall! at C:\projects\pycall-jl-nu3aa\src\pyfncall.jl:11 [inlined]
#pycall#110 at C:\projects\pycall-jl-nu3aa\src\pyfncall.jl:86 [inlined]
pycall at C:\projects\pycall-jl-nu3aa\src\pyfncall.jl:86
```

https://ci.appveyor.com/project/StevenGJohnson/pycall-jl-nu3aa/builds/23785003/job/rm9dhp093rs6ab8x#L99

Start with test/runtests.jl:29 and the __pycall! stack in src/pyfncall.jl:44, then review src/exception.jl:81. Reproduce the access violation on Windows x86 with the cited Julia and Python versions, compare against the last passing nightly, and confirm the regression is resolved when the Windows test suite passes.

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