Drop all unsupported versions

[yakky]

Dropped:

• Python 2
• Django < 2.2
• django CMS 3.6

Code compatibility layers to support python, old django will be removed in a future commit / PR

[coveralls]

Coverage increased (+97.2%) to 97.227% when pulling 5f4cd12e7ab8cc02422efb1da874fb2c9cdb3dde on feature/drop-python-2 into 20e3b474af78763a0a5264e9c1da31a8a2b3973c on develop.

[codecov[bot]]

Codecov Report

Merging #369 into develop will increase coverage by 1.22%. The diff coverage is 95.65%.

@@             Coverage Diff             @@
##           develop     #369      +/-   ##
===========================================
+ Coverage    91.56%   92.78%   +1.22%     
===========================================
  Files            7        7              
  Lines          640      610      -30     
  Branches       132      121      -11     
===========================================
- Hits           586      566      -20     
+ Misses          24       17       -7     
+ Partials        30       27       -3     

Impacted Files	Coverage Δ
djangocms_installer/config/__init__.py	91.92% <66.66%> (-0.48%)	:arrow_down:
djangocms_installer/config/data.py	100.00% <100.00%> (ø)
djangocms_installer/django/__init__.py	92.55% <100.00%> (+1.97%)	:arrow_up:
djangocms_installer/install/__init__.py	95.45% <100.00%> (+9.74%)	:arrow_up:
djangocms_installer/utils.py	100.00% <100.00%> (ø)

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