Ensure Python 3.7 support.

[tmontes]

Addresses #105.

[codecov-io]

Codecov Report

Merging #106 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #106   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          22     22           
  Lines         871    871           
  Branches       56     56           
=====================================
  Hits          871    871

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[tmontes]

Hmmm, Travis feedback is not coming through.

Motive:

• https://developer.github.com/changes/2018-11-05-github-services-brownout/
• Current integration is based on GitHub Services which are being deprecated.

Next:

• This needs investigation.

[tmontes]

Tweaked GitHub / Travis settings on both sides and we're back up and running. Cool.