chore(ci): introduce mypy

[nijel]

Proposed changes

Checklist

• Lint and unit tests pass locally with my changes.
• I have added tests that prove my fix is effective or that my feature works.
• I have added documentation to describe my feature.
• I have squashed my commits into logic units.
• I have described the changes in the commit messages.

Other information

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.32%. Comparing base (471d123) to head (4d8886e).
Report is 3 commits behind head on main.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2226      +/-   ##
==========================================
+ Coverage   80.29%   80.32%   +0.03%     
==========================================
  Files          34       34              
  Lines        3308     3309       +1     
  Branches      304      304              
==========================================
+ Hits         2656     2658       +2     
+ Misses        548      547       -1     
  Partials      104      104