Make all tests pass with django_dynamic_fixture 3.x

[stsewd]

When updated ddf in #6885, a lot of tests started to fail... the reason? We are creating models with fields they don't have.. ddf is more strict now, that's good. I didn't want to expend more time in #6885 so I'm creating this issue to keep track of that.

[santos22]

What's the reason these are failing? Are they all failing for the same reason? I tried running some tests locally and the following was what I got:

tox -e py37 readthedocs/rtd_tests/tests/test_project_forms.py

This might be because I'm using py37.

[stsewd]

@santos22 we are creating some models with invalid data, django dynamic fixture used to just ignore that, but with the new version it raises an exception. The codebase should run with python 3.6.

You can see some corrections in https://github.com/readthedocs/readthedocs.org/pull/6885/files

[santos22]

Sounds good. I'll try to get the codebase running with Python 3.6 and fix some of these tests.

[santos22]

Managed to get Python 3.6 running on my Windows machine. Working on making the tests pass 👍