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in Lib/ctypes/__init__.py the wstring_at and string_at functions are
declared with CFUNCTYPE.
This means that in Modules/_ctypes/callproc.c when the functions are
invoked, Py_UNBLOCK_THREADS and Py_BLOCK_THREADS surround the call. But
string_at and wstring_at call PyString_FromString and
PyUnicode_FromWideChar, respectively.
The solution (I think) is to declare the functions with PYFUNCTYPE
instead, so that callproc.c doesn't release the GIL when calling them.