as well as the tests of the task. Requires swh-scheduler >= 0.0.39

The pytest-cov is also moved in the requirements-test.txt file so that
we can depend on swh-scheduler[testing] which will handle the pytest<4
situation (so we won't have to fix each and every swh project when celery 4.3
is out and fixes this pytest incompatibility problem).

Otherwise, if we keep the dep on pytest-cov in the tox file, pip will not manage
properly the version constraints and install pytest>=4.