Add support for Django3

[godardth]

Directly import six library as it is not included anymore in Django utils.

[codecov]

Codecov Report

Merging #77 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #77   +/-   ##
=======================================
  Coverage   64.91%   64.91%           
=======================================
  Files          12       12           
  Lines         761      761           
=======================================
  Hits          494      494           
  Misses        267      267

Impacted Files	Coverage Δ
polymorphic_tree/models.py	63.24% <100%> (ø)	⬆️

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[vdboor]

Thanks for pointing this out! I've used your PR as a basis. The commit uses the future.utils library instead of adding six as it already contained the required functions :-)