Deprecate obsoletes and provides keywords

[smenon8]

As per comment, we also should consider deprecating obsoletes and provides keywords.

Please refer to issue #1374 for more details.

[pganssle]

To do on this ticket:

1. Understand the difference between Obsoletes and Obsoletes-Dist
2. Understand how that affects setuptools
3. Determine what the correct upgrade path is for both obsoletes and provides
4. Start raising warnings when either is specified
5. Add documentation to indicate that they are deprecated and how to migrate.

I believe obsoletes and provides are not currently used by pip or, to my knowledge, anything else, so we may want to note that in the documentation.

The relevant specs for the -Dist versions are here (two anchor links to the same page):

• Provides-Dist
• Obsoletes-Dist

The relevant spec for Obsoletes and Provides is found in PEP 314.