Manylinux support logic ignores overrides

[aragilar]

With the switch to use packaging.tags in pip 20, the values in _manylinux.py may be overridden by

packaging/packaging/tags.py

Line 660 in d49fdc5

# Support for a later manylinux implies support for an earlier version.

, which assumes that if a newer manylinux is supported, older ones must be too, and overrides the value (pip lacked this additional logic, hence why there were no previous issues). While this logic could be removed, a better solution would seem to be to have an explicit override (as contained in #262). This would seem to me to solve pypa/pip#3689.

There's an associated pip issue (pypa/pip#7648), which I created to track when this issue is fixed in pip.

[chrahunt]

As mentioned in #262, implementing manylinux_compatible as described in PEP 600 should do what we want. Note from the discussion page that this might not be straightforward.

[brettcannon]

#280 implemented PEP 600, so I believe this issue is taken care of. If I'm misunderstanding what is being asked then please let me know and we can reopen the issue.