Add Django 3.1 support

[FinalAngel]

• Added support for Django 3.1
• Dropped support for Python 2.7 and Python 3.4
• Dropped support for Django < 2.2

[codecov-commenter]

Codecov Report

Merging #117 into master will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##            master      #117    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           68        68            
  Lines         1286      1184   -102     
  Branches        87        62    -25     
==========================================
- Hits          1286      1184   -102     

Impacted Files	Coverage Δ
...ntrib/bootstrap4_alerts/migrations/0001_initial.py	100.00% <ø> (ø)
...ontrib/bootstrap4_badge/migrations/0001_initial.py	100.00% <ø> (ø)
...contrib/bootstrap4_card/migrations/0001_initial.py	100.00% <ø> (ø)
...rib/bootstrap4_carousel/migrations/0001_initial.py	100.00% <ø> (ø)
...s/0002_bootstrap4carousel_carousel_aspect_ratio.py	100.00% <ø> (ø)
...ap4_carousel/migrations/0003_auto_20180610_1102.py	100.00% <ø> (ø)
...rib/bootstrap4_collapse/migrations/0001_initial.py	100.00% <ø> (ø)
...cms_bootstrap4/contrib/bootstrap4_content/forms.py	100.00% <ø> (ø)
...trib/bootstrap4_content/migrations/0001_initial.py	100.00% <ø> (ø)
...bootstrap4_content/migrations/0002_added_figure.py	100.00% <ø> (ø)
... and 54 more

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