[Distutils] Adding Provides-Extra as an edit to PEP 345

[Daniel Holth]

> Given that PEP 345 is already in 'Accepted' state, I'm not sure we can
> just merge it in.  We might need to create a new PEP which extends PEP
> 345 and adds the feature (it likely needs to bump the 'Metadata-Version',
> too).

I would have thought so too, but then I downloaded 16,984 of the
newest sdists hosted on pypi, parsed their PKG-INFO or METADATA files,
and discovered adoption is quite low. Also, it is not Final.

 'Metadata-Version: 1.0': 15384,
 'Metadata-Version: 1.02': 1,
 'Metadata-Version: 1.1': 1383,
 'Metadata-Version: 1.2': 7,