Add Django 3 support

[hugodes]

These are some of the changes required by for compatibility with Django 3.0 ( #601 )

I don't know my way around the project so there are some tasks left to do.

• Support for python 2.7, and versions of Django << 2.0 have to be dropped.
• six has to be added as a dev
• tests for Django << 2.0 & Python << 3 have to be removed and replaced with tests for Django 3.0.

https://docs.djangoproject.com/en/3.0/releases/3.0/#features-deprecated-in-3-0
https://docs.djangoproject.com/en/3.0/releases/3.0/#removed-private-python-2-compatibility-apis

[hugodes]

Closing because a better PR is already made #600

[coveralls]

Coverage remained the same at ?% when pulling db43470 on hugodes:django-3 into 22ccd97 on jazzband:master.

[coveralls]

Coverage remained the same at ?% when pulling db43470 on hugodes:django-3 into 22ccd97 on jazzband:master.

[coveralls]

Coverage remained the same at ?% when pulling db43470 on hugodes:django-3 into 22ccd97 on jazzband:master.