Feature/django3 support

[slei99]

closes #132

[slei99]

used devel as source and target. Not sure if this suits your release strategy.

[coveralls]

Coverage remained the same at ?% when pulling b7dc4e4 on slei99:feature/django3-support into 86cb64b on bee-keeper:devel.

[slei99]

sorry, contrib and travis newbie. I expected this to run on the source branch. I dont get why this runs on devel branch and not the feature branch, this is why it fails (missing space in tox. ini r**_**requirements.txt).
On a different note, the travis.yml looks confusing to me with the env and ìnclude config. Just tried to add a resonable test config for django 3

[Siecje]

Does django-invitiations support Python2? It seems like six can be removed entirely.

[gtwohig]

@slei99 if you change your base branch for the PR it will run against the master branch, It would also be good to test against python 3.8 now that that is out.

@bee-keeper seems sound to remove python2 support altogether (like #134 ) and also to include the Django3 testing from this PR

[MrCordeiro]

Does django-invitiations support Python2? It seems like six can be removed entirely.

This makes total sense to me. With Python2 reaching the end of its support and Django 3 dropping the support altogether, it makes no sense to continue to support Python2 here either.