Removal of 'm' flag from tag in Python 3.8

[rdb]

Python 3.8 removes the existence of the m ABI flag:
https://bugs.python.org/issue36707
https://docs.python.org/dev/whatsnew/3.8.html#build-and-c-api-changes

Changes are needed in packaging.tags for platforms that do not define SOABI in the configuration. Linux and macOS wheels already are built without the m suffix. On Windows, pip fails to install binary wheels with the following error:

ERROR: mypackage-1.0.0-cp38-cp38-win_amd64.whl is not a supported wheel on this platform.

Fixing this is required to support installing binary Windows wheels in Python 3.8.

[rdb]

For the record, the ABI tag mechanism in packaging.tags does not suffer from this issue, as it reports the correct ABI tag, cp38. It is the pep425tags modules in wheel and pip (and maybe setuptools) that need fixing:

pypa/pip#6874
pypa/wheel#303
pypa/setuptools#1822

[brettcannon]

@rdb since packaging.tags is doing the right thing I'm going to close this issue, but thanks for noticing this and reporting it upstream as appropriate!