bpo-35502: Fix reference leaks in ElementTree.TreeBuilder. (GH-11170)

diff --git a/Modules/_elementtree.c b/Modules/_elementtree.c
index 12e418d..b1a9629 100644
--- a/Modules/_elementtree.c
+++ b/Modules/_elementtree.c
@@ -2447,6 +2447,11 @@
 static int
 treebuilder_gc_traverse(TreeBuilderObject *self, visitproc visit, void *arg)
 {
+    Py_VISIT(self->end_ns_event_obj);
+    Py_VISIT(self->start_ns_event_obj);
+    Py_VISIT(self->end_event_obj);
+    Py_VISIT(self->start_event_obj);
+    Py_VISIT(self->events_append);
     Py_VISIT(self->root);
     Py_VISIT(self->this);
     Py_VISIT(self->last);