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')]