Update no-warning tests for pytest-8

[mgorny]

Summary

Replace the deprecated pytest.warns(None) with the suggested replacement (from pytest-dev/pytest#9404) to make the test suite forward compatible with pytest-8. This works correctly with pytest-6 as well.

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
  • This change has an associated test.
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)

[mgorny]

Thanks!