Attempt on improving encoding in setuptools.tests

[abravalheri]

Summary of changes

Improve encoding warnings in files inside the setuptools/tests directory.

The following approaches were used:

1. Simply add encoding="utf-8"
2. Use open(..., "wb").close() instead of open(..., "w").close() when appropriate
3. Use jaraco.path.build instead of Path.write_text when appropriate.

According to #4234 (review) the changes that use approach (1) should be uncontroversial. I also believe that changes that use approach (2) are very trivial and make sense (because the main objective is to "touch" the file). But this PR does include some less trivial refactoring when going for approach (3).

Working towards the approach discussed in #4234 (review)

Pull Request Checklist

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

[abravalheri]

The non-trivial changes in this PR (like refactoring involving jaraco.path.build, mentioned as "approach (3)" above) are mainly to tests that I have written in the past, so even if they change a little bit the implementation detail, it should be fine.