Make state field readonly

changeset 49996de6e495 converted state field from Function field into Selection
field but it missed to properly keep it readonly because the state is managed
by update_state method.

issue6359
review26251002

purchase_request.py

@@ -85,7 +85,7 @@ class PurchaseRequest(ModelSQL, ModelView):
             ('draft', "Draft"),
             ('cancel', "Cancel"),
             ('exception', "Exception"),
-            ], "State", required=True, select=True)
+            ], "State", required=True, readonly=True, select=True)
 
     @classmethod
     def __setup__(cls):