DM-35565: Fix typing on ChoiceField

[natelust]

No description provided.

[codecov]

Codecov Report

Merging #83 (dc972a3) into main (1062784) will decrease coverage by 0.04%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##             main      #83      +/-   ##
==========================================
- Coverage   84.30%   84.25%   -0.05%     
==========================================
  Files          40       40              
  Lines        3172     3176       +4     
==========================================
+ Hits         2674     2676       +2     
- Misses        498      500       +2     

Impacted Files	Coverage Δ
python/lsst/pex/config/choiceField.py	76.92% <71.42%> (-4.90%)	⬇️

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