Fix CI: adjust type annotation for mypy

[puddly]

#187 wasn't updated with the new type annotations introduced in #224

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.32%. Comparing base (a9c9f8b) to head (5c8ea1a).
Report is 1 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #282   +/-   ##
=======================================
  Coverage   96.32%   96.32%           
=======================================
  Files          61       61           
  Lines        9380     9380           
=======================================
  Hits         9035     9035           
  Misses        345      345           

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