[1.8.x] Fixed #24940 -- Made model managers hashable

Thanks Federico Jaramillo Martínez for the report and Tim Graham for the
test and review.

Backport of d3d66d4722 from master

django/db/models/manager.py

@@ -236,6 +236,9 @@ class BaseManager(object):
     def __ne__(self, other):
         return not (self == other)
 
+    def __hash__(self):
+        return id(self)
+
 
 class Manager(BaseManager.from_queryset(QuerySet)):
     pass