Side effect of rDSCH3c8707599ae1cc721c5ee80b6def30e2374725b6.
The arrow package was installed in the tox venv through swh-scheduler
dependency. It is no more a scheduler dependency but should be installed
through swh-core[http] dependency. Unfortunately pip will install swh-core
first and will skip swh-core[http] due to its old resolver behavior.
So we need to explicitely add the arrow package as test requirement.