Add support for Django 3.1 / django CMS 3.8

[yakky]

Description

Add support for Django 3.1 / django CMS 3.8

Drop support for Python 3.5

References

Fix #376

Checklist

• [x] I have read the contribution guide
• [x] Code lint checked via inv lint
• [x] changes file included (see docs)
• [x] Usage documentation added in case of new features
• [x] Tests added

[codecov[bot]]

Codecov Report

Merging #378 (88c8ee7) into develop (ba8d611) will decrease coverage by 0.01%. The diff coverage is 88.88%.

@@             Coverage Diff             @@
##           develop     #378      +/-   ##
===========================================
- Coverage    94.46%   94.44%   -0.02%     
===========================================
  Files            7        7              
  Lines          614      612       -2     
  Branches       128      128              
===========================================
- Hits           580      578       -2     
  Misses          10       10              
  Partials        24       24              

Flag	Coverage Δ
unittests	94.44% <88.88%> (+0.14%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
djangocms_installer/config/__init__.py	97.20% <50.00%> (+0.03%)	:arrow_up:
djangocms_installer/config/data.py	100.00% <100.00%> (ø)

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[coveralls]

Coverage decreased (-0.006%) to 98.003% when pulling 88c8ee7d09cac106d9aaee5d946ef322f0cb329c on feature/issue-376-djangocms-38 into ba8d61159a568d3cdcb96084af934737a2b6eed8 on develop.