- Adds support for Celery v4.2. Technically this change is backward
incompatible because it is no longer possible to load tasks from bare modules
(e.g. mymodule.py in the Python root). This is a constraint imposed by Celery
v4.2. We however do not known of any cases where bare modules have been used,
and also this design is discouraged so we are not flagging it as a backward
incompatible change, in order to have the change readily available for
current Invenio version.