Issue #23685: Fix usage of PyMODINIT_FUNC in _json, _scproxy, nis, pyexpat

_codecs_cn, _codecs_hk, _codecs_iso2022, _codecs_jp, _codecs_kr and _codecs_tw
modules.

pyexpat.c doesn't need to redeclare PyMODINIT_FUNC, it's already declared in
Include/pyport.h.

Modules/_json.c

@@ -1995,7 +1995,7 @@ static struct PyModuleDef jsonmodule = {
         NULL
 };
 
-PyObject*
+PyMODINIT_FUNC
 PyInit__json(void)
 {
     PyObject *m = PyModule_Create(&jsonmodule);