Improve codebase to python3

[sevdog]

Remove python2 super and drop useless method redefinitions.

[Natim]

Thank you @sevdog

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 3 lines in your changes missing coverage. Please review.

Project coverage is 85.55%. Comparing base (338e171) to head (ff5073d).
Report is 26 commits behind head on master.

Files with missing lines	Patch %	Lines
django_downloadview/files.py	50.00%	1 Missing ⚠️
django_downloadview/nginx/middlewares.py	50.00%	1 Missing ⚠️
django_downloadview/storage.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #207      +/-   ##
==========================================
+ Coverage   85.51%   85.55%   +0.03%     
==========================================
  Files          72       72              
  Lines        1312     1308       -4     
==========================================
- Hits         1122     1119       -3     
+ Misses        190      189       -1     

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[sevdog]

I just came across this and was forced to do a couple of updates to better handle my usecase (see https://github.com/sevdog/django-downloadview/tree/updated), I tried to keep single changes to small PRs. If anything else is welcome I will arrange new PRs.

[tari]

Nobody seems to have objected to these changes and I think they're good to have, so I'm going to go ahead and merge this.