Add exception class to pytest.warns calls

[QuLogic]

PR Summary

This is failing on current pytest.

PR Checklist

Tests and Styling

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (install flake8-docstrings and run flake8 --docstring-convention=all).

Documentation

• [n/a] New features are documented, with examples if plot related.
• [n/a] New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• [n/a] API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).
• [n/a] Documentation is sphinx and numpydoc compliant (the docs should build without error).

[oscargus]

Maybe add a match for the two without one?

[QuLogic]

Those are checked in the above test_register_cmap, I think, so I didn't add the match, but could.