Add __hash__ method for permissions.OperandHolder class (#9417)

`OperandHolder` is not hashable, so need to add `__hash__` method

rest_framework/permissions.py

@@ -54,6 +54,9 @@ def __eq__(self, other):
             self.op2_class == other.op2_class
         )
 
+    def __hash__(self):
+        return hash((self.operator_class, self.op1_class, self.op2_class))
+
 
 class AND:
     def __init__(self, op1, op2):