Add Python 3.6 to testing

[cclauss]

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• I've read and agree to the Code of Conduct.
• I've been mindful about doing atomic commits, adding documentation to my changes, not refactoring too much.
• I've a descriptive title and added any useful information for the reviewer. Where appropriate, I've attached a screenshot and/or screencast (gif preferably).
• I've written tests to cover the new code and functionality included in this PR.
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PR Summary

e.g. New functionality for producing whatsits.

Related Issues

e.g. Fixes #206 and closes #230

Test strategy

e.g. Add tests around whatsit production.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #176 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #176   +/-   ##
=======================================
  Coverage   64.16%   64.16%           
=======================================
  Files           9        9           
  Lines         293      293           
=======================================
  Hits          188      188           
  Misses        105      105

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[Roach]

Thanks, @cclauss 👍