Rewrite the celery task as a decorated function
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.
Migrated from D966 (view on Phabricator)