Upgrade to Django>=2.2

[benmurden]

• Add tox and pytest for easier testing and coverage reports
  • django-coverage is no longer maintained
• Make new migrations
• Add on_delete=models.PROTECT to foreign keys.
• Remove compatibility code for Django<2.2.
• Update importlib from Django version to Python standard lib.
• Update urllib2 to Python 3 standard lib.
• Update encoding for Mixpanel task.
• Change print statements to Python 3 style.
• Change test files to test_* naming convention.
• Add manage.py for easier testing with tox and pytest.
• Remove django-celery. No longer needed with modern Celery.
• Remove mock and use Python 3 unittest.mock.
• Update statsd implementation and tests.
• Update management command base class.
• Add timezone support. Replaces #42.
• Update test settings.
• Exclude tests from package.
• Update test requirements.

[benmurden]

@frankwiles Have you had a chance to look at this? Anything I can help with?