Fix tox for Python3.6 drop support

[ahopkins]

Removes Python 3.6 from tox testing envs.

[codecov]

Codecov Report

Merging #2037 (dec46d5) into master (6b86921) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2037   +/-   ##
=======================================
  Coverage   92.06%   92.06%           
=======================================
  Files          33       33           
  Lines        3102     3102           
  Branches      542      542           
=======================================
  Hits         2856     2856           
  Misses        166      166           
  Partials       80       80           

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