Release v2.4 - #964

@albertyw thank you for this, it helps! Your other PR about cleaning up supported versions got me thinking that this should be named 3.0 to mark the python 2 compatibility drop.

In terms of the sequencing of #964, #965, and #966, I think that:

1. Fix tests #966 should be merged first to get the build working again
2. Release v2.4 #964 merged as release v2.4 to get compatibility with django 3.0
3. Remove support for python 2.7, 3.4 and django 2.1 #965 merged to break backwards compatibility by not supporting older versions of python/django
4. A PR made that removes code specifically for python 2 support (i.e. removes the dependency on six)
5. Another PR then created/merged/released as django-compressor v3.0.

This allows people to get Django 3.0 support without dealing with backwards-incompatibilities.

I'd be fine with that, although to claim django 3.0 support for django-compressor 2.4. In that regard, it would indeed help if you split that out of that other PR so that could be merged before #964. But since @diox started the discussion I'd leave the decision to him.

I liked the idea of having compressor 3.0 coinciding with Django 3.0 support, but this wasn't based on anything rational, it just looked cool :) Let's follow the plan proposed by @albertyw in #964 (comment).

please also see my other comment here: #965 (comment)

To be clear, I agree with @karyon that #965 should be split so that the Django 3.0 support is added without any extra cleanups. Then we merge this one, then release, then do the extra cleanups, including removing Django 1.11 and Python 2 for our 3.0 release.

great! I can help with any of that if need be.

So as an update to PR sequencing from #964 (comment):

1. Fix tests #966 has been merged so the base build works no2
2. Remove django.utils.six #969 should be merged to remove the last mention of django.utils.six
3. Update flake8 to be compatible with newer versions of python #968 updates flake8 to make tests compatible with python 3.8.
4. Test against python 3.8 and django 3.0 #967 starts testing against python 3.8 and django 3.0. However, a third party dependency is blocking there.
5. Update django-appconf #978 to make sure all dependencies are compatible with django 3
6. Release v2.4 #964 for release 2.4
7. Remove six
8. Remove support for python 2.7, 3.4 and django 2.1 #965 to remove EOL python and django versions
9. Cleanup code with Switch from smart_text to smart_str #977 and Remove six #976
10. ??
11. Release 3.0

I added a commit with tweaks to the release notes, as far as I'm concerned they are good to go now. @diox could you check?

With regards to the confusion around #957, it seems like that was squash-merged before 2.3 already, but the contributor continued to work on a branch with the un-squashed commits and included those in a later PR, which we merged as well, but that time without squash-merging. So tl;dr our git history is a little messed up but that does not concern the 2.4 release notes :)

Thanks for the release!

thanks. see the discussion in #963