tests: replace nose with pytest

We do use the pytest-django plugin to manage the django db setup magic, so with this change, we can run tests without an existing db, or starting a postgres db dedicated to the tests using pifpaf.

related to swh-environment#1261 (closed)

also:

• replace assertEquals -> assertEqual

• qdd a tox file

  We need to explicitely specify the location where pytest looks for tests to prevent the kind of nasty error described here:

  https://tox.readthedocs.io/en/latest/example/pytest.html#known-issues-and-limitations

  Also note that migrations scripts are excluded from flake8 in the tox file.

related to swh/infra/ci-cd/swh-jenkins-jobs#1247 (closed).

Test Plan

tox should be happy, hopefully jenkins will also

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Migrated from D592 (view on Phabricator)