Add support for Django 3.1

[sliverc]

Fixes #822

Description of the Change

Add support for Django 3.1

Checklist

• PR only contains one change (considered splitting up PR)
• unit-test added
• documentation updated
• CHANGELOG.md updated (only for user relevant changes)
• author name in AUTHORS

[n2ygk]

Looks good. I'll rebase #772 (again) after I merge this.

[codecov]

Codecov Report

Merging #835 into master will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #835      +/-   ##
==========================================
- Coverage   97.08%   97.08%   -0.01%     
==========================================
  Files          58       58              
  Lines        3158     3157       -1     
==========================================
- Hits         3066     3065       -1     
  Misses         92       92              

Impacted Files	Coverage Δ
example/tests/test_errors.py	100.00% <100.00%> (ø)
example/tests/test_parsers.py	100.00% <100.00%> (ø)
example/urls_test.py	100.00% <100.00%> (ø)

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