CPython 3.8 debug/release wheel ABI compatibility

[chrahunt]

Per bpo-36465, Python 3.8 will support libraries compiled against a release build running in a debug build of the interpreter.

Pip could make this easier to take advantage of by allowing e.g. cp38dm and cp38m wheels in a debug Python 3.8. If we are planning to switch to packaging.tags then that may be the place to make the actual changes and this issue could be closed or track the integration.

Additional context

• When wheel ABI tag doesn't require debugging, but debugging is enabled, wheel is not instaleld pip#5733
• python-dev mailing list by @vstinner

[pradyunsg]

Moved to packaging since we'd want to do this in packaging.tags, so that there's no duplicated effort.