chore: eat Python 2 leftovers

[pajod]

• No harm in merge conflicts: can always be trivially regenerated thanks to https://pypi.org/project/pyupgrade/ with its --py37-plus flag
• This is an updated version of refactor: remove python 2 new-style classes as only python 3.5+ is supported now #2836

[benoitc]

why removing this line? can't we still have files using latin 1 or other encoding?

[pajod]

Removing the utf-8 comment because it does not do anything. https://peps.python.org/pep-3120/

And if we were to want files in latin-1:

• Either they are Python sources, then it needlessly complicated to not have them in UTF-8
• Or they are not, then PEP 263 is unfit to signal that, because editors would at best get that right for Python sources.

Fortunately, the only place we could possibly want that is in tests/requests/valid/*.py.. and there, we never did and should not start now. If you like, I will rewrite the tests to even change affected .http files (example) to unambiguous 7-bit-clean format, e.g. by using\N{Latin Capital Letter A with Diaeresis} escapes.