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

[Daniel Holth]

> +1 To this.  I agree Setup-Requires and Provides-Extra are different
> use cases and should both be added to the spec.  I think that filling
> in these two missing features will benefit packaging in the long run.
> If this idea gains some amount of acceptance I'm happy to work on an
> implementation of the setup-requires end of things for packaging.

One last addition, explicitly allow X- headers. IMO the technical
purpose of each tag is solid. I would like to get this implemented by
the end of July so that it can be used.

After that I would like to open PEP 346 "Metadata 1.3". Metadata 1.3 could add:

Parenthesis in environment markers (make official "a subset of Python
without most of the good stuff?")

The ~> operator for version specs "~> 1.2" is sugar for ">= 1.2, <
2.0". (Not allowed in environment markers). Thanks dstufft.

Metadata-Is-Static: true / false (name to be determined; means "does
Requires-Dist reliably use environment markers, or does it depend on
Python code in setup.py)