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.

Objects/structseq.c

@@ -579,7 +579,8 @@ int _PyStructSequence_Init(void)
     if (_PyUnicode_FromId(&PyId_n_sequence_fields) == NULL
         || _PyUnicode_FromId(&PyId_n_fields) == NULL
         || _PyUnicode_FromId(&PyId_n_unnamed_fields) == NULL)
+    {
         return -1;
-
+    }
     return 0;
 }