nox: readthedocs broken

Describe the bug

The docs are broken, the “stable” release did not build after the recent release (missing from changelog), with:

AttributeError: 'HTMLParser' object has no attribute 'unescape'

See https://readthedocs.org/projects/nox/builds/

This should be fixable by setting a minimum version on some of the docs dependencies (why are those mixed with the test dependencies?) - readthedocs installs old versions of things by default, and then it’s a no-op if you don’t force an upgrade. Why they install a version of (something that uses HTML) that doesn’t support Python 3.9+, I’m not sure, but forcing sphinx>=4 or something like that will likely fix the problem. Downgrading to Python 3.8 for the docs would fix it, too, but that’s the wrong way to initially try to fix it.

I would highly recommend enabling PR readthedocs builds (in the readthedocs settings), they are very handy for catching bugs in the docs builds.

Also, it looks like “latest” hasn’t built in a year or more, I’m guessing it was never changed to main from master, perhaps? It makes “latest” much older than “stable”.