chore: drop Python 3.9 (and syntax) (#6289)

* `strict=False` is the default, pyupgrade to min version 3.10 adds this
to be explicit w/ behavior

libs/prebuilt/tests/test_react_agent_graph.py

@@ -1,4 +1,4 @@
-from typing import Callable, Union
+from collections.abc import Callable
 
 import pytest
 from pydantic import BaseModel
@@ -38,9 +38,9 @@ class ResponseFormat(BaseModel):
 def test_react_agent_graph_structure(
     snapshot: SnapshotAssertion,
     tools: list[Callable],
-    pre_model_hook: Union[Callable, None],
-    post_model_hook: Union[Callable, None],
-    response_format: Union[type[BaseModel], None],
+    pre_model_hook: Callable | None,
+    post_model_hook: Callable | None,
+    response_format: type[BaseModel] | None,
 ) -> None:
     agent = create_react_agent(
         model,