Misc. typing fixes

[benbovy]

Many typing issues were not caught by mypy (untyped unit tests).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.15%. Comparing base (38a0693) to head (b07515d).
Report is 39 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #84      +/-   ##
==========================================
- Coverage   94.00%   88.15%   -5.85%     
==========================================
  Files           5       13       +8     
  Lines         200      785     +585     
  Branches        8       61      +53     
==========================================
+ Hits          188      692     +504     
- Misses          5       63      +58     
- Partials        7       30      +23     

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[benbovy]

I have no idea why CI passed in 20a5836 and not in 3ad4104 (both using mypy version <1). Anyway, using mypy 1.x seems to work in the last commit (I've run the workflow multiple times to make sure it is not flaky).