Adopt more UTF-8

[abravalheri]

Summary of changes

Previously, changes regarding UTF-8 avoided touching complicated parts of the code where it could cause incompatibility.

This PR is a bit more aggressive, but still try to maintain backwards compatibility:

• When reading files, first try to read them as UTF-8, then we fallback to the locale encoding.
• When writing files, use UTF-8.

In my opinion the highest risk here are in the easy_install/install_scripts commands, because it might be the case some scripts are not meant to be UTF-8... But hopefully that risk would be minimal[^1].

[^1]: Once file contents are encoded as Python strings they are already encoded as UTF-8, so it should be OK to write them directly to files. Moreover, most popular scripting languages nowadays support UTF-8.

Closes

Pull Request Checklist

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

[PR docs]: https://setuptools.pypa.io/en/latest/development/developer-guide.html#making-a-pull-request

[abravalheri]

@jaraco, please let me know if you are OK with a more aggressive move towards UTF-8. (in this PR I am still trying to avoid breaking changes by adding some fallback, but one never know the edge cases)