[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

tests/generic_inline_admin/models.py

@@ -48,7 +48,7 @@ class PhoneNumber(models.Model):
 
 class Contact(models.Model):
     name = models.CharField(max_length=50)
-    phone_numbers = GenericRelation(PhoneNumber)
+    phone_numbers = GenericRelation(PhoneNumber, related_query_name='phone_numbers')
 
 
 #