Add support for maintainer in PKG-INFO

[pganssle]

Fixes #1288.

I have not really gone through this with a fine-toothed comb to see if it meets all of the specifications of PEP 345, but this is at least a step in the right direction.

[pganssle]

Oops! This is an artifact.

[fungi]

Looks like you dropped checking for project_urls when moving the sieve from write_pkg_file() to get_metadata_version(), where it was previously setting a minimum metadata version of 1.2 per the Project-URL entry for PEP 345. Was that intentional, or an oversight? I'm just trying to figure out whether I should push a PR to readd the check for project_urls or whether it wasn't really necessary in the first place.

[pganssle]

@fungi I believe the reason that I dropped checking for project_urls because PEP 345 specifies that both Requires-Python and Project-URL are required, and so anything using version >= 1.2 must have Requires-Python.

That said, this is a heuristic based on metadata generated mostly from setup(), so probably the original checking for "if you request any features of 1.2, we should create a valid version 1.2+ file".

So it was intentional but probably wrong. Feel free to make a PR or issue to add it back.

[fungi]

Thanks, will do. For the record, I found this specifically because I noticed after a new setuptools release I was suddenly ending up with PKG-INFO files claiming to be metadata version 1.1 compliant even though they included Project-URL entries.