Register lister tasks even if they do not derive from SWHTask.

This happens if swh.scheduler.celery_backend.config isn't imported
before the CLI runs.

swh/lister/cli.py

@@ -8,12 +8,12 @@ import logging
 from copy import deepcopy
 from importlib import import_module
 
+import celery.app.task
 import click
 from sqlalchemy import create_engine
 
 from swh.core.cli import CONTEXT_SETTINGS
 from swh.scheduler import get_scheduler
-from swh.scheduler.task import SWHTask
 from swh.lister import get_lister, SUPPORTED_LISTERS, LISTERS
 from swh.lister.core.models import initialize
 
@@ -124,7 +124,7 @@ def register_task_types(ctx, listers):
             mod = import_module(task_module)
             for task_name in (x for x in dir(mod) if not x.startswith('_')):
                 taskobj = getattr(mod, task_name)
-                if isinstance(taskobj, SWHTask):
+                if isinstance(taskobj, celery.app.task.Task):
                     task_type = task_name.replace('_', '-')
                     task_cfg = registry_entry.get('task_types', {}).get(
                         task_type, {})