pin sphinx temporarily while debugging doc build

[dhalbert]

Sphinx 8.1.0 just came out today. It breaks the latexpdf build, and also causes new messages like:

checking consistency... /home/halbert/repos/circuitpython/BUILDING.md: document is referenced in multiple toctrees: ['docs/index', 'docs/pdf'], selecting: docs/pdf <- BUILDING
/home/halbert/repos/circuitpython/CODE_OF_CONDUCT.md: document is referenced in multiple toctrees: ['docs/index', 'docs/pdf'], selecting: docs/pdf <- CODE_OF_CONDUCT
/home/halbert/repos/circuitpython/CONTRIBUTING.md: document is referenced in multiple toctrees: ['docs/index', 'docs/pdf'], selecting: docs/pdf <- CONTRIBUTING
...

I haven't been able to reduce this to a small case to file an issue with sphinx, but I've added a discussion reply here:
https://github.com/orgs/sphinx-doc/discussions/12999#discussioncomment-10909269

[tannewt]

Thank you for debugging this!