Allow NewType.__supertype__ to also be a NewType (#11728)

stdlib/typing_extensions.pyi

@@ -374,7 +374,7 @@ else:
     class NewType:
         def __init__(self, name: str, tp: Any) -> None: ...
         def __call__(self, obj: _T, /) -> _T: ...
-        __supertype__: type
+        __supertype__: type | NewType
         if sys.version_info >= (3, 10):
             def __or__(self, other: Any) -> _SpecialForm: ...
             def __ror__(self, other: Any) -> _SpecialForm: ...