Fix __reduce__ signature

Author: cetanu

src/betterproto/__init__.py

@@ -1140,7 +1140,7 @@ def __setstate__(self: T, pickled_bytes: bytes) -> T:
         new.__post_init__()
         return new
 
-    def __reduce__(self) -> str | tuple[Any, ...]:
+    def __reduce__(self) -> Union[str, Tuple[Any, ...]]:
         return (self.__class__, (self.to_dict(),))
 
     @classmethod