DEP: Support Python 3.13

[larsoner]

Closes #13013

• Add CI run for 3.13
• Get pytest mne/utils to pass locally
• Get all tests passing
• Check all copied docs to make sure they render okay (yikes)

I already know some do not because pytest mne/tests/test_docstring_parameters.py fails. But I want to make sure it fails here, and see what else breaks 🙈

[larsoner]

Spot checked the docs including evoked.plot_topo and pushed a little commit to work around openmeeg/openmeeg#700 on Windows, marking for merge-when-green to get CIs back happy again but @drammock feel free to look and I'm happy to open a follow-up PR with more changes if needed.