Run clean with PEP-597 `PYTHONWARNDEFAULTENCODING=1`, add to selftests

[Zac-HD]

I recently tried checking for encoding issues in a downstream project, but the first thing to fail was in Pytest itself. I therefore think that we should enable this check for our selftests in CI, and ensure that we're handling encodings correctly per the link above.

[The-Compiler]

Duplicate of #10326? Also see #10328.