Add support for Django 3.0.x

[stphivos]

Fix builds to merge PR #107

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@f05ccca). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #109   +/-   ##
=========================================
  Coverage          ?   99.18%           
=========================================
  Files             ?        6           
  Lines             ?      857           
  Branches          ?        0           
=========================================
  Hits              ?      850           
  Misses            ?        7           
  Partials          ?        0

Impacted Files	Coverage Δ
django_mock_queries/asserts.py	95.94% <100%> (ø)

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