Fix inconsistent sphinx versions

[aucampia]

We are currently stuck on an old sphinx in pyproject.toml because of incompatibilities with flake8 and flakeheaven, but we use a different sphinx on readthedocs, so we can get nitpicky mode to work correctly. This is causing lots of pain because we can't run offline with the same setup that we use on readthedocs easily, which makes it hard to get docs to build successfully.