Add note on the nature of the workaround

Author: hartwork

Lib/test/test_pyexpat.py

@@ -909,6 +909,12 @@ def test_set_activation_threshold__fail_for_subparser(self):
         subparser = parser.ExternalEntityParserCreate(None)
         setter = functools.partial(self.set_activation_threshold, subparser)
         self.assert_root_parser_failure(setter, 12345)
+
+        # NOTE: This is a workaround for issue
+        #       https://github.com/python/cpython/issues/139400
+        #       that can be dropped once pull request
+        #       https://github.com/python/cpython/pull/139403
+        #       has been merged.
         del setter
         del subparser
         del parser