Resolve deprecated features for django3.2

[powderflask]

I think there's only one deprecated item - that is easy to fix.
Will also need to update tox to include tests for django3.2 and python 3.8 / 3.9

Consider removing support for python2.x, < python3.7, <django2.2 ?

I am assembling a pull request - let me know if you'd like me to include removing support for unsupported versions, otherwise I'll just add support for django3.2 and python3.8, 3.9

[powderflask]

One challenge: dependency persisting_theory may be abandoned? It also needs a few tweaks...

[agateblue]

I'll handle persisting_theory myself, likely next week, I'll keep you updated here :)

[agateblue]

@powderflask I've bumped a new version of persisting_theory (https://code.agate.blue/agate/persisting-theory), it should be compatible with python 3 without any warning, do you mind giving it a try?

[powderflask]

@agateblue this works great! thanks!