Enable disallow_untyped_defs for mypy

[atugushev]

Closes #972 🎉

[codecov]

Codecov Report

Merging #1350 (e6055a3) into master (852edf8) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1350   +/-   ##
=======================================
  Coverage   99.65%   99.65%           
=======================================
  Files          33       33           
  Lines        2915     2915           
  Branches      308      308           
=======================================
  Hits         2905     2905           
  Misses          5        5           
  Partials        5        5           

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[jdufresne]

Yes!