Sphinx cleanups, bump version requirements, typecheck docs source files

[jakkdl]

I was struggling with sphinx in #2898 and noticed that sphinx was pinned to an ancient version, and unpinning & updating it seemed to fix a couple of them. My struggles also made me understand sphinx innards better than I had before, so could fix a bunch of references at the same time.

• add missing type annotations to docs/source/*.py
• specify files to check with mypy in pyproject.toml (docs/source/typevars.py was typed already, but not checked in CI)
• change sphinx version limits to >5.3
• clean up & fix a bunch of references in sphinx
• add comment explaining workarounds

let's see if RTD manages this as well

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d4ce2f9) 99.49% compared to head (52ddc6c) 99.66%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2909      +/-   ##
==========================================
+ Coverage   99.49%   99.66%   +0.16%     
==========================================
  Files         115      115              
  Lines       17704    17667      -37     
  Branches     3158     3157       -1     
==========================================
- Hits        17615    17608       -7     
+ Misses         70       40      -30     
  Partials       19       19              

see 4 files with indirect coverage changes

[TeamSpen210]

Might want to mention the typevar module also, which redirects typing_extensions → typing, and all typevars to typing.TypeVar etc.

[CoolCat467]

This looks good to me. I'm hopeful this will help with working with sphinx in the future, it's errors are confusing to me!