Switch to pytest-django.
Refactor Amazon API connection out of model manager into utility function.
Move get_transitively_similar movies from model method to utility function.
Move helper decorators for tests into test package.
Add a more robust set of tests on transitive movie relationships.
Move Amazon API exception handling to utility function.
Add tests on Amazon API connection using mock.
Assert that Amazon API helper returns None on error.

This replaces the old behvior, which needlessly returned
a namedtuple filled with Nones.
Pass updated unit tests by expecting None when API breaks.
Add tests on model managers.
Add tests on models.
Switch to pytest-cov instead of straight coverage.
Split model managers into separate module.