Maintenance updates - add support up to Django 4.0

[Stormheg]

With this PR we test the package against Django 2.2 up to Django 4.0 with all Django-supported Python versions.

• This includes Python 3.6 which is end-of-life since last year, do we want to drop support?
• We continue testing against Django 3.0 and 3.1, even though those versions are not supported anymore. Do we want to stop testing against those versions and thus, drop support?

Further updates include:

• Fixed the failing tests (SECRET_KEY not set error and test_contib.py::test_check_cache_default_down test failing)
• Reformat using black 22.3
• Some testing dependency upgrades
• Use a more recent pytest version that supports Python 3.10
• Remove the need for the six library since we don't support Python 2 anymore
• Update the changelog to reflect all of the above changes

[denisSurkov]

@Stormheg , hi!

Are there any thoughts to merge it?

[Stormheg]

Hey @denisSurkov 👋

Not sure if this will be merged any time soon. This PR has been open for quite a while with no activity.

If you are looking for Django 4.0 support, it looks like that already made it and was released in v1.5.0.

[denisSurkov]

@Stormheg , thank you! That's exactly what I was looking for.