Test out sphinx-autodoc-typehints extension

[dgw]

I have an idea on my list from ages ago to see if the sphinx-autodoc-typehints extension improves our docs.

For one thing, it could improve our parameter lists. It might also help with the warnings Sphinx currently spits out for some of the type hints in signatures scattered around, but that seems unlikely at best. (The extension appears to add :type: and :rtype: annotations to the rST source before Sphinx renders it, which wouldn't affect how the problematic types are resolved. Most likely we need to wait on sphinx-doc/sphinx#10785 and/or sphinx-doc/sphinx#10974 to actually fix those warnings.)

Don't want to spend time on this before 8.0.0 is out, but also don't want to forget about it once we start on 8.0.1. Documentation tweaks are great candidates for maintenance releases.