Add a new SWHTask class to be used as base class for swh celery tasks

It is meant to be used to declare swh tasks via the task decorator instead of subclassing the (now deprecated) Task class.

It is typically used like:

from swh.scheduler.celery_backend.config import app from swh.scheduler.tasks import SWHTask

@app.task(base=SWHTask) def ping(): return 'pong'

Also replace the TaskRouter class by a simple function class-based router is a remainder of celery 3.

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Migrated from D886 (view on Phabricator)