Ignore newlines when processing keywords/platforms

[frenzymadness]

Summary of changes

Newlines in keywords or platforms can break
the produced metadata in PKG-INFO or METADATA files.

Fixes: #4887

Pull Request Checklist

• Changes have tests
• News fragment added in [newsfragments/].

[abravalheri]

Thank you very much @frenzymadness for having a look on this issue and proposing a solution.

I think we have 2 options according to https://setuptools.pypa.io/en/latest/development/developer-guide.html#code-conventions-and-other-practices (see description about modifying files in the _distutils folder):

1. Contribute this change to the pypa/distutils repository (requires changing the test cases to another file that exists in the pypa/distutils repository).
2. Attempt to implement a different change outside of the _distutils folder.

In the case we go for a change in the pypa/setuptools repository, I believe that the best here is to comply with the documented behaviour and raise an error when \n is used (maintainers of pypa/distutils might have a different opinion).

[frenzymadness]

You are right, sorry. Let me take this to distutils and see what they think about this change there.