[entities] add a CeleryTask.parent_tasks property

that returns an iterator on all parents recursively

entities.py

@@ -121,6 +121,13 @@ class CeleryTask(AnyEntity):
         for subtask in self.reverse_parent_task:
             yield subtask.progress
 
+    @property
+    def parent_tasks(self):
+        yield self
+        for task in self.parent_task:
+            for ptask in task.parent_tasks:
+                yield ptask
+
 
 class ICeleryTask(EntityAdapter):
     __regid__ = 'ICeleryTask'