Prepare for Sphinx 8.x

[dylanh724]

Context

Sphinx 8.x is here -- PyData is already preparing:

https://github.com/pydata/pydata-sphinx-theme/issues/1938

Proposal

Follow PyData's lead.

Tasks and updates

• [ ] Keep an eye on PyData's Sphinx 8 progress + their next release progress
• [ ] Ensure compatibility
• [ ] Prepare for new release

[dylanh724]

Hmm, has anyone tried just upgrading to 8 and see if it magically worked without issues?