[Python PSM Interop] Use FrozenSet (#35759)

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Closes #35759

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/35759 from XuanWang-Amos:python_ssa_client_test_part_2 86bd6b1ed5
PiperOrigin-RevId: 602820831

src/python/grpcio_tests/tests_py3_only/interop/xds_interop_client.py

@@ -23,6 +23,7 @@ import time
 from typing import (
     DefaultDict,
     Dict,
+    FrozenSet,
     Iterable,
     List,
     Mapping,
@@ -84,7 +85,7 @@ class _StatsWatcher:
     _no_remote_peer: int
     _lock: threading.Lock
     _condition: threading.Condition
-    _metadata_keys: frozenset
+    _metadata_keys: FrozenSet[str]
     _include_all_metadata: bool
     _metadata_by_peer: DefaultDict[
         str, messages_pb2.LoadBalancerStatsResponse.MetadataByPeer