Python 3 compatibility

[driesdesmet]

We are going to start deploying python 3 django websites. Make sure this works.

[RubenVW-dev]

there is no incompatible code yet

[driesdesmet]

It should be checked regularly and even better is to put a testing mechanism in place to check wether django projects based on this cookiecutter produce python 3 compatible code. Reopening for now.