update dep on swh-scheduler>0.0.39 and pytest<4 (tests)
pytest<4 because of https://github.com/pytest-dev/pytest/issues/4641
Migrated from D963 (view on Phabricator)
pytest<4 because of https://github.com/pytest-dev/pytest/issues/4641
Migrated from D963 (view on Phabricator)