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

[Daniel Holth]

Add to metadata 1.3:

Description-File: README(\..+)?

Meaning the description should be read from a file in the same
directory as PKG-INFO or METADATA (including in the .dist-info
directories) and we strongly recommend it be named as README.* and be
utf-8 encoded text.

Description: is the only multi-line field in the metadata. It is
almost never needed at runtime. It would be great for performance and
simplify the parser to just put it in another file.

Mutually exclusive with Description.

May beg for a Summary: tag with a one-line description.