bpo-45459: C API uses type names rather than structure names (GH-31528)

Thanks to the new pytypedefs.h, it becomes to use type names like
PyObject rather like structure names like "struct _object".

Include/structseq.h

@@ -15,7 +15,7 @@ typedef struct PyStructSequence_Field {
 typedef struct PyStructSequence_Desc {
     const char *name;
     const char *doc;
-    struct PyStructSequence_Field *fields;
+    PyStructSequence_Field *fields;
     int n_in_sequence;
 } PyStructSequence_Desc;