Fix for SF bug #485678.

slot_tp_descr_set(): When deleting an attribute described by a
descriptor implemented in Python, the descriptor's __del__ method is
called by the slot_tp_descr_set dispatch function.  This is bogus --
__del__ already has a different meaning. Renaming this use of __del__
is renamed to __delete__.

Objects/typeobject.c

@@ -3281,7 +3281,7 @@ slot_tp_descr_set(PyObject *self, PyObject *target, PyObject *value)
 	static PyObject *del_str, *set_str;
 
 	if (value == NULL)
-		res = call_method(self, "__del__", &del_str,
+		res = call_method(self, "__delete__", &del_str,
 				  "(O)", target);
 	else
 		res = call_method(self, "__set__", &set_str,