PR Summary

pydata-sphinx-theme updated, renaming some configuration names.
(only one of which we actually configure)
While they are just warnings for now, we are configured to error on
warning so CircleCI is indicating failure.

This does the 'easy' route of just changing to the new name and expecting
the version in which it is implemented.

If desired, I could do a larger change to accomodate both by checking
the version, but I'm also nottoo worried about more tightly pinning
already relatively tight requirements.

PR Checklist

Documentation and Tests

• [N/A] Has pytest style unit tests (andpytest passes)
• Documentation is sphinx and numpydoc compliant (the docs shouldbuild without error).
• [N/A] New plotting related features are documented with examples.

Release Notes

• [N/A] New features are marked with a.. versionadded:: directive in the docstring and documented indoc/users/next_whats_new/
• [N/A] API changes are marked with a.. versionchanged:: directive in the docstring and documented indoc/api/next_api_changes/
• [N/A] Release notes conform with instructions innext_whats_new/README.rst ornext_api_changes/README.rst