bpo-40217: Ensure Py_VISIT(Py_TYPE(self)) is always called for PyType_FromSpec types (reverts GH-19414) (GH-20264)

Heap types now always visit the type in tp_traverse. See added docs for details.

This reverts commit 0169d3003b.

Automerge-Triggered-By: @encukou

Python/Python-ast.c

@@ -1109,6 +1109,7 @@ ast_dealloc(AST_object *self)
 static int
 ast_traverse(AST_object *self, visitproc visit, void *arg)
 {
+    Py_VISIT(Py_TYPE(self));
     Py_VISIT(self->dict);
     return 0;
 }