Add mypy & fix display module so it passes

[charles-turner-1]

Adds mypy.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.32%. Comparing base (5bda03a) to head (6283645).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #87      +/-   ##
==========================================
+ Coverage   94.10%   94.32%   +0.22%     
==========================================
  Files           6        6              
  Lines         424      423       -1     
==========================================
  Hits          399      399              
+ Misses         25       24       -1     

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