[Bugfix] Python 3.10 compatibility for Self (#27918)

Signed-off-by: DarkLight1337 <tlleungac@connect.ust.hk>

vllm/config/structured_outputs.py

@@ -2,10 +2,11 @@
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 
 import hashlib
-from typing import Any, Literal, Self
+from typing import Any, Literal
 
 from pydantic import model_validator
 from pydantic.dataclasses import dataclass
+from typing_extensions import Self
 
 from vllm.config.utils import config