Fix: Drop support for Python 3.6, 3.7 and Django 2.2

[adam-murray]

Dropping support for old/no longer maintained versions of Python and Django. Please see the 4.0.x branch for those still wishing to use Django 2.2 or Python <3.8.

Also switching from djangocms_app_helper to django_app_helper.

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@a98d71a). Click here to learn what that means. The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #85   +/-   ##
=========================================
  Coverage          ?   77.82%           
=========================================
  Files             ?       30           
  Lines             ?      780           
  Branches          ?       76           
=========================================
  Hits              ?      607           
  Misses            ?      153           
  Partials          ?       20