Remove deprecated code after warnings have been issued for a long period.

[abravalheri]

Summary of changes

• Remove setuptools.convert_path
• Remove fallback for customisations of distutils' build.sub_command
• Re-enable enforcement via tox so that the maintainers of setuptools are "notified" when the next deprecation comes[^1]

The deprecation warnings have been issued for almost 2 years for these features.

Closes

Pull Request Checklist

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

[^1]: By "notified", I mean the CI builds will start to fail and require manual intervention...