Testing: linter, add flake8-return

[rdimaio]

See: https://docs.astral.sh/ruff/rules/#flake8-return-ret

[rdimaio]

There's a few rules here (like RET505: https://docs.astral.sh/ruff/rules/superfluous-else-return/) that I think should be first discussed with the other developers, as there's arguments on both sides. On one hand, as the else is superfluous, we should remove it to simplify the code; on the other hand, having the else might make the code more readable.