[Python-Dev] Edits to Metadata 1.2 to add extras (optional dependencies)

[Daniel Holth]

After this discussion it seemed wiser to submit my proposed 1.2 edits
as Metadata 1.3, adding Provides-Extra, Setup-Requires-Dist, and
Extension (with no defined registration procedure). This version is
sure to be exciting as it also specifies that the values are UTF-8
with tolerant decoding and re-defines environment markers in terms of
the ast module (is there a better way to specify a subset of Python?).

The proposed Metadata 1.3 is at
https://bitbucket.org/dholth/python-peps/changeset/8fa1de7478e95b5ef3a18c3272f740d8f3e2fb80

Thanks,

Daniel Holth