Repair Descriptions with newlines and emit a warning

[jaraco]

Closes #1390

Pull Request Checklist

• Changes have tests
• News fragment added

[alexshpilkin]

Nit: maybe it’d also make sense to str.rstrip() broken descriptions? My guess is that most descriptions with newlines have one at the end. If careless usage of multiline strings (see e.g. PyLops/pyproximal@90d37d0) is frequent enough, it might also make sense to handle newlines and spaces at the beginning as well, and so str.strip() the descriptions instead.

[jaraco]

Good point, although I do want to be cautious and not overcorrect here. If somehow a space is already present at the beginning or end of the description, I wouldn't want to alter that as part of correcting for newlines, even if that is the correct thing to do. On the other hand, converting a trailing or leading newline to a space seems janky at best.

I think for now I'll stick with this approach, as it still will correct the underlying defect (that metadata gets corrupted) and provide a path forward. I'll defer more sophisticated validation to subsequent efforts (which probably should happen earlier than when writing values out and probably coordinated across multiple forms of input such as setup.py or setup.cfg).