Also test python 3.6 on github actions.

[mcmtroffaes]

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[codecov]

Codecov Report

Merging #294 (fb6625b) into develop (1d44990) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           develop      #294   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           48        48           
  Lines         2345      2345           
  Branches       411       411           
=========================================
  Hits          2345      2345           

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