Support Django 3 and Python 3.8

[patrys]

• Drop support for Python 3.4
• Add support for Python 3.8
• Drop support for Django <2.2
• Add support for Django 3.0
• Replace django-babel (dead) with enmerkar (currently supported fork)

[codecov[bot]]

Codecov Report

Merging #94 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #94   +/-   ##
=======================================
  Coverage   89.76%   89.76%           
=======================================
  Files           6        6           
  Lines         127      127           
  Branches       11       11           
=======================================
  Hits          114      114           
  Misses         10       10           
  Partials        3        3

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