Add django-4.0 and Drop django-3.1

[shimizukawa]

Feature or Bugfix

• Feature

Purpose

• Add django-4.0
• Drop django-3.1
• Testing with djmain (It hasn't worked with github actions until now)

[codecov]

Codecov Report

Merging #98 (1dd9dbb) into master (2e824ff) will increase coverage by 1.01%.
The diff coverage is 82.35%.

@@            Coverage Diff             @@
##           master      #98      +/-   ##
==========================================
+ Coverage   62.99%   64.00%   +1.01%     
==========================================
  Files           4        4              
  Lines         427      439      +12     
  Branches      122      127       +5     
==========================================
+ Hits          269      281      +12     
  Misses        116      116              
  Partials       42       42              

Impacted Files	Coverage Δ
django_redshift_backend/base.py	63.23% <82.35%> (+1.11%)	⬆️

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