mgr/volumes: `threading.Event.isSet` no longer exists in py3

mgr/volumes: `threading.Event.isSet` no longer exists in py3

fixes mypy errors:

volumes/fs/async_job.py: note: In member "should_cancel" of class "JobThread":
volumes/fs/async_job.py:70: error: "Event" has no attribute "isSet"

volumes/fs/volume.py: note: In member "is_stopping" of class "VolumeClient":
volumes/fs/volume.py:59: error: "Event" has no attribute "isSet"

volumes/fs/volume.py: note: In member "list_fs_volumes" of class "VolumeClient":
volumes/fs/volume.py:106: error: "Event" has no attribute "isSet"

Fixes: https://tracker.ceph.com/issues/44393
Signed-off-by: Michael Fritch <mfritch@suse.com>