chore(types): Configure and run mypy checks in CI

[effigies]

As a relatively small package, this seemed like a good place to start. As we add type annotations, mypy will ensure that we do it correctly.

cc @jhlegarreta

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.32%. Comparing base (d923a42) to head (cccd528).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #146      +/-   ##
==========================================
+ Coverage   65.10%   65.32%   +0.22%     
==========================================
  Files          25       25              
  Lines        2642     2642              
  Branches      415      415              
==========================================
+ Hits         1720     1726       +6     
+ Misses        811      807       -4     
+ Partials      111      109       -2     

Flag	Coverage Δ
unittests	64.79% <100.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[jhlegarreta]

Thanks for doing this @effigies. I have only one inline question.

I see checks passing 🎉.