turn PyErr_WarnPy3k into a macro

Python/_warnings.c

@@ -775,15 +775,6 @@ PyErr_WarnExplicit(PyObject *category, const char *text,
 }
 
 
-int
-PyErr_WarnPy3k(const char *text, Py_ssize_t stacklevel)
-{
-    if (Py_Py3kWarningFlag)
-        return PyErr_WarnEx(PyExc_DeprecationWarning, text, stacklevel);
-    return 0;
-}
-
-
 PyDoc_STRVAR(warn_doc,
 "Issue a warning, or maybe ignore it or raise an exception.");