fix 3.9 test with union syntax

Author: sydney-runkle

tests/test_prebuilt.py

@@ -1,3 +1,5 @@
+from typing import Union
+
 from pydantic_core import SchemaSerializer, SchemaValidator, core_schema
 
 
@@ -55,7 +57,7 @@ class InnerModel:
         def __init__(self, x: str) -> None:
             self.x = x
 
-    def serialize_inner(v: InnerModel, serializer) -> dict[str, str] | str:
+    def serialize_inner(v: InnerModel, serializer) -> Union[dict[str, str], str]:
         v.x = v.x + ' modified'
         return serializer(v)