bpo-36623: Clean parser headers and include files (GH-12253)

After the removal of pgen, multiple header and function prototypes that lack implementation or are unused are still lying around.
diff --git a/Parser/listnode.c b/Parser/listnode.c
index 71300ae..8f1a116 100644
--- a/Parser/listnode.c
+++ b/Parser/listnode.c
@@ -1,7 +1,7 @@
 
 /* List a node on a file */
 
-#include "pgenheaders.h"
+#include "Python.h"
 #include "token.h"
 #include "node.h"