[types] Make UnionType.__getitem__ parameter positional-only (#15458)

Author: brianschubert

stdlib/types.pyi

@@ -733,7 +733,7 @@ if sys.version_info >= (3, 10):
         def __hash__(self) -> int: ...
         # you can only subscript a `UnionType` instance if at least one of the elements
         # in the union is a generic alias instance that has a non-empty `__parameters__`
-        def __getitem__(self, parameters: Any) -> object: ...
+        def __getitem__(self, parameters: Any, /) -> object: ...
 
 if sys.version_info >= (3, 13):
     @final