Fix test failures caused by warns(None).

[maksverver]

With pytest>=7.0, warns(None) fails with:

TypeError: exceptions must be derived from Warning, not <class 'NoneType'>

The recommended solution is to use warnings.simplefilter("error") instead. See: https://docs.pytest.org/en/latest/how-to/capture-warnings.html#additional-use-cases-of-warnings-in-tests

What does this PR do?

Fixes broken tests.

Before submitting

• Did you have fun?
  • Not really
• Did you read the contributor guideline?
  • Yes
• Was this discussed/approved via a Github issue? (no need for typos, doc improvements)
  • No
• Did you make sure to update the docs?
  • N/A
• Did you write any new necessary tests?
  • N/A
• Did you update the changelog? (if needed)
  • No, because I didn't change any production code, only tests.

PR review

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