bpo-43687: Py_Initialize() creates singletons earlier (GH-25147)

Reorganize pycore_interp_init() to initialize singletons before the
the first PyType_Ready() call. Fix an issue when Python is configured
using --without-doc-strings.

Python/errors.c

@@ -1192,7 +1192,7 @@ static PyStructSequence_Desc UnraisableHookArgs_desc = {
 
 
 PyStatus
-_PyErr_Init(void)
+_PyErr_InitTypes(void)
 {
     if (UnraisableHookArgsType.tp_name == NULL) {
         if (PyStructSequence_InitType2(&UnraisableHookArgsType,