Use Django 2.0 only when using Python 3
Use Django 2.0 only when using Python 3

Django 2.0 is not compatible with Python 2. Indicate this in Django
dependency when running Python 2.

While at it, force using Python >= 3.4.2 when using Python 3, like
before commit 51a3e7f78514 ("Make the project compatible with Python
2.7")

Polytechnique-org/xorgauth,Polytechnique-org/xorgauth