Don't call setuptools.extern.importlib_metadata.EntryPoints.names for better backwards compatibility.

[abravalheri]

Summary of changes

• Don't call setuptools.extern.importlib_metadata.EntryPoints.names, instead just iterate over entry_points (this is the same underlying implementation, but it is backwards compatible).

Closes #4338

Pull Request Checklist

• Changes have tests
• News fragment added in newsfragments/.
  (See documentation for details)

[abravalheri]

Merging based on this comment:

Ideally, I'd like for setuptools to be able to rely on whatever version of "importlib metadata" is available in the environment.

(As it implies that setuptools should be able to deal with older importlib_metadata installations.)