Drop support for Python 2.7 and Django < 2.0

[jorenham]

This removes the six dependency, many of the compat.py functions, and a lot of other code, that now has become irrelevant.

[W1ldPo1nter]

Thanks, this certainly has to be done at some point. However, I'm going to leave this open for now as I don't plan to drop the Python 2.7 support just yet (for the simple reason that I still have some legacy projects to tend to, which benefit from the library).

[jorenham]

@W1ldPo1nter Legacy projects will still be able to use django-queryable-properties just fine. They just won't be able to use the newer versions. This is what all major projects have done, including Django, and even Python (since august 2020).

[W1ldPo1nter]

Sure, my point was that these legacy projects would also benefit from further updates to the library (which can easily be upgraded even in such projects due to having basically no dependencies). Quite a few fixes I've made to django-queryable-properties were based on bugs that I've encountered by using the library in those projects.

As stated before, I do plan to drop support for older versions at some point, so your work won't be in vain. I just don't want to do it just yet, which is why I'm leaving this PR open to be merged at a later date.

[wolph]

Both Django and Python stopped support for 2.7 roughly 4 years ago.

There have been multiple security issues in Django since that time so I do hope that your legacy projects are not Internet facing :)