add default None -> UserWarning

[lilyminium]

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Fixes #4746

Changes made in this Pull Request:

• This mimics the Python warnings library by setting category=None to UserWarning by default

We don't appear to test our utilities anywhere so I haven't added a formal test -- but running it with #4744 worked.

PR Checklist

• [ ] Tests?
• [ ] Docs?
• [ ] CHANGELOG updated?
• [x] Issue raised/referenced?

Developers certificate of origin

• [x] I certify that this contribution is covered by the LGPLv2.1+ license as defined in our LICENSE and adheres to the Developer Certificate of Origin.

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📚 Documentation preview 📚: https://mdanalysis--4747.org.readthedocs.build/en/4747/