Add mypy to CI flow

[huangsam]

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Add mypy to CI flow.

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See whether things will work or break. And tweak as needed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (ce764d3) to head (c7e9100).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #142   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           36        36           
  Lines         1153      1153           
  Branches        62        62           
=========================================
  Hits          1153      1153           

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