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

[Daniel Holth]

And some stats: of the 16k sdists I surveyed, 9484 would need to use
'Requires-Dist:', and 1624 define what would be a Provides-Extra:
line.

I found 483 unique extra names.

So Provides-Extra: really has nothing to do with tests or docs. I'm
sorry I included that distracting remark in the specification.
Provides-Extra: is all about requiring dependencies that enable an
optional feature of a distribution.

Some of the most popular extra names:

[(5, 'kid'),
 (5, 'lxml'),
 (5, 'oracle'),
 (5, 'pylons'),
 (5, 'simplejson'),
 (5, 'wsgi'),
 (5, 'xml'),
 (6, 'all'),
 (6, 'django'),
 (6, 'jinja2'),
 (6, 'nose'),
 (7, 'sphinx'),
 (8, 'mysql'),
 (9, 'doc'),
 (10, 'zcml'),
 (12, 'app'),
 (12, 'zopesupport'),
 (13, 'dev'),
 (20, 'genshi'),
 (22, 'mako'),
 (41, 'testing'),
 (60, 'docs'),
 (305, 'tests'),
 (1053, 'test')]