Add six

[MHM5000]

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when you use git clone and then run pip install -r requirements.txt, six is available as a dependency of one of the packages, but when you do a pip install django-river (or in my case pipenv install django-river) and include river in the INSTALLED_APPS; upon running the server you'll see an error related to six not being available.

This PR fixes that.

[MHM5000]

django-codemirror2 needs to be added here and in INSTALLED_APPS but I have to go now. I'll add it tomorrow.