Research PT009 for Django assertions

[m-burst]

During the implementation of #218 I discovered that some unittest-style assertions we're checking against are actually not from unittest.TestCase but from Django's SimpleTestCase (namely assertNotContains and assertRaisesMessage).

We should possibly research which of the Django assertions can be simply replaced by simple assert statements or replaced with a pytest-idiomatic plugin, and add a rule that would warn about usage of these assertions.

[jenstroeger]

I’m working with a Django repo and that’s what I ran into. For now, I’m going to disable that check just to be on the safe side.

Also, any chance you’d consider an “auto-fix” option, that rewrites the code? (Not sure flake8 would allow that, but…)