parsers: return NULL from PyInit_parsers on Python 3

This function must return a PyObject* or the compiler complains.

mercurial/parsers.c

@@ -2880,7 +2880,7 b' PyMODINIT_FUNC PyInit_parsers(void)'
 	PyObject *mod;
 
 	if (check_python_version() == -1)
-		return;
+		return NULL;
 	mod = PyModule_Create(&parsers_module);
 	module_init(mod);
 	return mod;