Without this, some dependencies from swh.core[http] are not installed
(because swh.core, without extras, is installed first from
requirements-swh.txt).

Probably similar to this issue https://github.com/pypa/pip/issues/6506

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 T1247.