chore: enable mypy check `warn_return_any`

chore: make code PEP597 compliant

test(functional): bump GitLab docker image to 15.2.0-ee.0

Use `encoding="utf-8"` in `open()` and open-like functions.

https://peps.python.org/pep-0597/

Update code so that the `warn_return_any` check passes.

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