Add testing for Django 2.1

[timgraham]

This will have to wait until after the Django 2.1 beta release as the test failures will be fixed by https://github.com/django/django/pull/10003. (now done)

[benkonrath]

Great, thanks!

[codecov-io]

Codecov Report

Merging #335 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #335   +/-   ##
=======================================
  Coverage   95.89%   95.89%           
=======================================
  Files         153      153           
  Lines        3851     3851           
  Branches      512      512           
=======================================
  Hits         3693     3693           
  Misses         96       96           
  Partials       62       62

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[benkonrath]

Thanks again.