Handle ABI compatibility on Windows, Python 2.x and other implementations

[qwcode]

The current ABI compatibility tagging in PEP 425 relies on distutils/sysconfig reporting a correct answer for SOABI. This isn't valid on Windows or on Python 2.x.

The updated PEP needs to cover how to derive an appropriate ABI compatibility tag on Windows and Python 2.x, as well as providing more clarity around handling of alternative implementations when they offer an FFI that isn't just an emulation of the CPython C API.

---

migrated from https://bitbucket.org/pypa/pypi-metadata-formats/issues/25/handle-abi-compatibility-on-windows-python