Bugfix to add Django 3 compatibility

[rhenter]

Add Django 3 support.

(https://docs.djangoproject.com/en/3.0/releases/3.0/#removed-private-python-2-compatibility-apis)

Follows the error below that was fixed:

$ python manage.py migrate jet
Traceback (most recent call last):
  File "manage.py", line 22, in <module>
    execute_from_command_line(sys.argv)
  File ".../.virtualenvs/supplai-api/lib/python3.6/site-packages/django/core/management/__init__.py", line 401, in execute_from_command_line
    utility.execute()
  File ".../.virtualenvs/supplai-api/lib/python3.6/site-packages/django/core/management/__init__.py", line 377, in execute
    django.setup()
  File ".../.virtualenvs/supplai-api/lib/python3.6/site-packages/django/__init__.py", line 24, in setup
    apps.populate(settings.INSTALLED_APPS)
  File ".../.virtualenvs/supplai-api/lib/python3.6/site-packages/django/apps/registry.py", line 114, in populate
    app_config.import_models()
  File ".../.virtualenvs/supplai-api/lib/python3.6/site-packages/django/apps/config.py", line 211, in import_models
    self.models_module = import_module(models_module_name)
  File ".../.pyenv/versions/3.6.10/Python.framework/Versions/3.6/lib/python3.6/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 994, in _gcd_import
  File "<frozen importlib._bootstrap>", line 971, in _find_and_load
  File "<frozen importlib._bootstrap>", line 955, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 665, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 678, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File ".../lib/python3.6/site-packages/jet/models.py", line 3, in <module>
    from django.utils.encoding import python_2_unicode_compatible
ImportError: cannot import name 'python_2_unicode_compatible'

[coveralls]

Coverage remained the same at 52.381% when pulling f1934c7a6feaabe76994f40adfb17014e2a7d493 on rhenter:dev into 06ab6436d8add9aafcf771df40358409564e6bcb on geex-arts:dev.