Bumped version to 2.3.0

[FinalAngel]

• Added support for Django 2.2 and django CMS 3.7
• Removed support for Django 2.0
• Extended test matrix
• Added isort and adapted imports
• Adapted code base to align with other supported addons

[codecov-io]

Codecov Report

Merging #41 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #41   +/-   ##
=======================================
  Coverage   84.48%   84.48%           
=======================================
  Files          13       13           
  Lines         174      174           
  Branches       10       10           
=======================================
  Hits          147      147           
  Misses         26       26           
  Partials        1        1

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