Django 3 compatibility: django.utils.six has been removed

[MarkusShepherd]

Django 3 removed some Python 2 compatibility libraries which this project depends on, making it impossible right now to upgrade to Django 3.

This PR patches the problem by catching the ImportError and importing six directly instead. It also adds six as a dependency (in the version as it was available in Django 2.2).

A full fix would be to drop six altogether since Python 2 is officially retired now and no longer supported by Django, but this is beyond the scope of this PR.

[codecov-io]

Codecov Report

Merging #33 into master will decrease coverage by 0.74%. The diff coverage is 50%.

@@            Coverage Diff             @@
##           master      #33      +/-   ##
==========================================
- Coverage   67.55%   66.81%   -0.75%     
==========================================
  Files           5        5              
  Lines         225      229       +4     
==========================================
+ Hits          152      153       +1     
- Misses         73       76       +3

Impacted Files	Coverage Δ
rest_framework_proxy/views.py	69.78% <50%> (-1.33%)	:arrow_down:

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