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

[Daniel Holth]

>> Add Provides-Extra: to PEP 345 and clarify Requires-Dist: name (1) is the same as Requires-Dist: name (==1)

> +1

Great. How do we get https://bitbucket.org/dholth/python-peps merged
into hg.python.org/peps/ ?