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'