Introduce mypy

[wkeeling]

Fixes #352

[codecov-commenter]

Codecov Report

Merging #388 (3ef0448) into master (cf45862) will not change coverage.
The diff coverage is n/a.

❗ Current head 3ef0448 differs from pull request most recent head 72d562f. Consider uploading reports for the commit 72d562f to get more accurate results

@@           Coverage Diff           @@
##           master     #388   +/-   ##
=======================================
  Coverage   80.36%   80.36%           
=======================================
  Files          14       14           
  Lines        1085     1085           
=======================================
  Hits          872      872           
  Misses        213      213           

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