Fix PEP 728 TypedDict.__closed__ annotation in typing_extensions (#14839)

Author: cdce8p

stdlib/typing_extensions.pyi

@@ -239,7 +239,7 @@ class _TypedDict(Mapping[str, object], metaclass=abc.ABCMeta):
     __readonly_keys__: ClassVar[frozenset[str]]
     __mutable_keys__: ClassVar[frozenset[str]]
     # PEP 728
-    __closed__: ClassVar[bool]
+    __closed__: ClassVar[bool | None]
     __extra_items__: ClassVar[AnnotationForm]
     def copy(self) -> Self: ...
     # Using Never so that only calls using mypy plugin hook that specialize the signature