Update pythoncapi_compat.h
Update pythoncapi_compat.h

Fix compatibility with Visual Studio 2008 for Python 2.7:
* https://phab.mercurial-scm.org/D9867
* https://github.com/pythoncapi/pythoncapi_compat/pull/3

indygreg/python-zstandard,indygreg/python-zstandard,indygreg/python-zstandard,indygreg/python-zstandard