Support for Django 3.0

[kamilturek]

This PR introduces the support for Django 3.0. It also introduces a tox.ini config to verify all the supported Python & Django versions.

Closes #56, #64, #35, #37.

Setup & all tests pass on following versions:

• Python >= 3.6, <= 3.10
• Django >= 2.2, < 3.3

Tox output:

___________________________________ summary ____________________________________
  py36-django22: commands succeeded
  py36-django30: commands succeeded
  py36-django31: commands succeeded
  py36-django32: commands succeeded
  py37-django22: commands succeeded
  py37-django30: commands succeeded
  py37-django31: commands succeeded
  py37-django32: commands succeeded
  py38-django22: commands succeeded
  py38-django30: commands succeeded
  py38-django31: commands succeeded
  py38-django32: commands succeeded
  py39-django22: commands succeeded
  py39-django30: commands succeeded
  py39-django31: commands succeeded
  py39-django32: commands succeeded
  py310-django22: commands succeeded
  py310-django30: commands succeeded
  py310-django31: commands succeeded
  py310-django32: commands succeeded