Remove Validator base class, fix hidden type error

[marcpaterno]

This will close issue #377 . It also fixes a type error that was made manifest when the tighter type signatures were put into place.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (481a6bc) 96.2% compared to head (8cee81e) 96.2%.
Report is 1 commits behind head on master.

❗ Current head 8cee81e differs from pull request most recent head 8070355. Consider uploading reports for the commit 8070355 to get more accurate results

Additional details and impacted files

@@          Coverage Diff           @@
##           master    #378   +/-   ##
======================================
  Coverage    96.2%   96.2%           
======================================
  Files          35      35           
  Lines        2569    2573    +4     
======================================
+ Hits         2472    2476    +4     
  Misses         97      97           

Files	Coverage Δ
firecrown/connector/mapping.py	100.0% <100.0%> (ø)
firecrown/descriptors.py	100.0% <100.0%> (ø)