Test updates

[hovi]

Following #207 , I started playing with the project and getting tests running:

• Updates tox file so that it works with version 4
  https://tox.wiki/en/4.0.3/faq.html#tox-4-changed-ini-rules
• Updates test_reqs.txt to be consistent with tox.ini (versions and packages in test_reqs.txt were outdated)
• Adds django manage.py file pointing to test tests.test_settings config file.

First 2 were breaking my test runs.
3rd one allows running tests simply by running python manage.py test in console - so "django way" of running tests. That means I can run every single test method by simply clicking icon in PyCharm, yay!

I haven't dived deeper into runtests.py and post_processor_runtests, but from first glance it seems like these could be fully removed - 1st by just doing python manage.py test and the second by running it with --testrunner tests.post_processor.discover_tests.DiscoverPostProcessorRunner.

[hovi]

No idea why the coverage upload isn't passing. I noticed that in job, there is tox>=3,<4, otherwise I don't see anything that could affect it.