Sdists (again): Metadata standardisation incremental update

[xafer (Xavier Fernandez)]

@ncoghlan

Switching topics from the opt in spelling, there’s one “must be static” field that I think requires further clarification (both here and in PEP 621): Requires-Python

I’m assuming the intent is that platform specific Python version dependencies should be expressed through environment markers, but neither PEP spells that out.

Remember that Requires-Python currently does not support environment markers (cf specifications: Requires-Python doesn't accept an environment marker by xavfernandez · Pull Request #771 · pypa/packaging.python.org · GitHub for context).