Django 3.0 Compatibility

[kavdev]

It'd be nice to get django-choices compatible with django 3.0 until we can provide:

1. options on how to achieve parity with the new native choices classes in django 3.0
2. a forward migration path (with documentation) from django-choices to django native choices

See also https://github.com/bigjason/django-choices/issues/65

Things to fix:

• [x] django 3.0 dropped their vendored version of six (release notes).

[sergei-maertens]

Indeed - I need to set some time aside for this. Thanks for listing up the tasks, either way!

I've merged your PR, I'll see if I can push it to PyPI later today 😃