Pylint's `bad-classmethod-argument` is disabled for each definition
to avoid rewriting the method bodies just to rename `self` → `cls`.
This can be done gradually, as the setUpTestData methods are modified
along the way.
This way, we need not list every value again to create the enum.
N.B.: enum values are now accessed as `models.NotificationType.FOO`,
instead of `models.Notification.FOO`.