Make an error message more understandable and consistent with other error messages.

Objects/abstract.c

@@ -156,7 +156,7 @@ PyObject_GetItem(PyObject *o, PyObject *key)
                               "be integer, not '%.200s'", key);
     }
 
-    return type_error("'%.200s' object is not subscriptable", o);
+    return type_error("'%.200s' object has no attribute '__getitem__'", o);
 }
 
 int