Match return type of SerializationInfo.mode() to mode of model_dump()

Pydantic in some place use Literal['python', 'json'] | str the
serialization mode, using a more restrictive type can break stuff (e.g.
typing validation). So this commit adapt the return type to make it
matches with other pydantic valid serialization mode.

Author: Julian Vanden Broeck

python/pydantic_core/core_schema.py

@@ -145,7 +145,7 @@ def context(self) -> ContextT:
         ...
 
     @property
-    def mode(self) -> Literal['python', 'json']:
+    def mode(self) -> Literal['python', 'json'] | str:
         """The serialization mode set during serialization."""
         ...