[windows] Support Py_LIMITED_API again

Created originally on Bitbucket by ofekmeister (Ofek Lev)

Changesets for this Pull Request have not been imported, because it had been already declined on Bitbucket. Marked as closed by the import user.

Related to

https://bitbucket.org/cffi/cffi/issues/355/importerror-dll-load-failed-on-windows

https://bitbucket.org/cffi/cffi/issues/350/issue-with-py_limited_api-on-windows

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A new version of virtualenv has been released that includes the long-awaited fix https://pypi.org/project/virtualenv

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Please let me know if more changes are required! cc @arigo @anthrotype @reaperhulk