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%)	⬇️

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Codecov Report

Attention: Patch coverage is 50.00000% with 3 lines in your changes missing coverage. Please review.

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Files	Patch %	Lines
rest_framework_proxy/views.py	50.00%	3 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff            @@
##             master      #33   +/-   ##
=========================================
  Coverage          ?   66.81%           
=========================================
  Files             ?        5           
  Lines             ?      229           
  Branches          ?        0           
=========================================
  Hits              ?      153           
  Misses            ?       76           
  Partials          ?        0           

Files	Coverage Δ
rest_framework_proxy/views.py	69.78% <50.00%> (ø)

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1fa51b1...3b9d838. Read the comment docs.