Add comment about Py_CLEAR(self->parent) to xmlparse_clear

Author: hartwork

Modules/pyexpat.c

@@ -1545,6 +1545,11 @@ xmlparse_clear(PyObject *op)
     xmlparseobject *self = xmlparseobject_CAST(op);
     clear_handlers(self, 0);
     Py_CLEAR(self->intern);
+    // NOTE: We cannot call Py_CLEAR(self->parent) prior to calling
+    //       XML_ParserFree(self->itself) or a subparser (created via
+    //       XML_ExternalEntityParserCreate could lose its parent XML_Parser
+    //       while still making use of it internally.
+    //       https://github.com/python/cpython/issues/139400
     return 0;
 }