Pytest warnings fix

[kulikjak]

Summary of changes

Pytest recently deprecated passing None as pytest.warns argument. As per the documentation [1], new preferred way of ensuring that no warnings are emitted is:

with warnings.catch_warnings():
    warnings.simplefilter("error")

I verified that this change doesn't work only with the latest Pytest (7.0.0 at this time) but also with much older 4.4.0.

[1] https://docs.pytest.org/en/latest/how-to/capture-warnings.html#additional-use-cases-of-warnings-in-tests

Pull Request Checklist

• Changes have tests (changes are tests!)
• Authors have been added to AUTHORS.md
• News fragment added in changelog.d. See CONTRIBUTING.md for details - I presume it's not necessary in this case?

[pganssle]

Great, thanks!