Drop support for unsupported Python and Django versions

[jaap3]

As mentioned in issue #535 there was already the intention to drop Python 2 support. I've prepared a branch that removes compatibility shims and updates the code to a more modern style.

You can see the result here: https://github.com/leukeleu/django-imagekit/pull/3

Note that I merged the branches for PR #538, #539 and #534 in there as well.

I can rebase that branch as soon as those other PRs are merged. I tried to make the commits on that branch as standalone as possible. So if there are things you'd rather not have changed I can easily remove those commits.

[vstoykov]

I looked at your PR in your repo and the only thing that bothered me was the changes around pkgmeta.py. If this metadata was moved to __init__.py and everything is working correctly we do not need it anymore, It's idea was to get the metadata in the setup.py without the execution of the __init__.pywhich is also trying to import some other stuff. I like the idea of modernizing the setup.py and the inclusion of pyproject.toml but I'm not sure about the loading of the metadata this way.

[jaap3]

I believe I kept the pkgmeta.py file as it's used in the docs (which I didn't really touch).

I won't have time to work on this for a couple of weeks, so I'll have to put this on hold. When I get some time I'll probably undo some of the packaging changes and hold them back for a separate pull request. This branch got somewhat out of control with all the changes :-)

[vstoykov]

I totally understand you. Sorry if somehow you feel pressure or you spend too much time on this project. The reality is that I haven't had enough time in the last 3 years and because finally there was someone willing to spend some time to contribute code and I can't miss that chance :).

I appreciate your work and I'm thankful for it. When you have more time it will be more than welcome.

[jaap3]

I've created PR #544 to drop compatibility shims and use modern Python language features.

I've omitted the packaging changes as I felt those are out of scope for this ticket and wasn't 100% sure about those anyway.