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

Modules/_curses_panel.c

@@ -39,6 +39,7 @@ _curses_panel_clear(PyObject *m)
 static int
 _curses_panel_traverse(PyObject *m, visitproc visit, void *arg)
 {
+    Py_VISIT(Py_TYPE(m));
     Py_VISIT(get_curses_panelstate(m)->PyCursesError);
     return 0;
 }