Add Python 3.7 Support

[ahawker]

If merged, this will include Python 3.7 as a supported version
for the package.

[codecov]

Codecov Report

Merging #283 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #283   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           5      5           
  Lines         267    267           
  Branches       58     58           
=====================================
  Hits          267    267

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