Let mypy find unused ignores

[mattwthompson]

Just seeing if/how much this differs from my local setup okay, this ended up working out well.

Basically does what the change in the config file implies, saving some human effort like I saw in #1374. I have this in most of my projects and have been happy with it.

Checklist

• Added a news entry

Developers certificate of origin

• I certify that this contribution is covered by the MIT License here and the Developer Certificate of Origin at https://developercertificate.org/.

[github-actions]

No API break detected ✅

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.24%. Comparing base (04387d7) to head (03a8824).
⚠️ Report is 121 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1423      +/-   ##
==========================================
- Coverage   95.11%   92.24%   -2.88%     
==========================================
  Files         165      165              
  Lines       12389    12389              
==========================================
- Hits        11784    11428     -356     
- Misses        605      961     +356     

Flag	Coverage Δ
fast-tests	92.24% <100.00%> (?)
slow-tests	?

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[mikemhenry]

LGTM!