unbreak doc build with Sphinx 6.2

[rcomer]

PR Summary

Sphinx v6.2 was released yesterday, and the nitpick_ignore config variable may now be a set, tuple or list, but defaults to set. This broke our extension, which assumes the variable is a list (as it was previously). See e.g. the most recent commit on main for example failure.

I have also failed to get anything newer than Sphinx v5.3 in environments created from our conda yml, so a solution that also works for older versions is desirable.

PR Checklist

Linked Issue

• Added "closes #0000" in the PR description to link it to the original issue.

Documentation and Tests

• Has pytest style unit tests (and pytest passes)
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• New plotting related features are documented with examples.

Release Notes

• New features are marked with a .. versionadded:: directive in the docstring and documented in doc/users/next_whats_new/
• API changes are marked with a .. versionchanged:: directive in the docstring and documented in doc/api/next_api_changes/
• Release notes conform with instructions in next_whats_new/README.rst or next_api_changes/README.rst

[ksunden]

Cross ref sphinx-doc/sphinx#11355

(I had started working on a solution similar to this, but you got the PR in as I was running the doc build locally)

If this were an extension we provided for downstream use, I may suggest maintaining the input type (if only to ensure compatibility with their own extensions which could rely on a particular data structure in much the same way) but since this is really only for internal use, no need to over complicate.

[tacaswell]

anyone can merge when CI is green.