Adjust stubtest test stubs for PEP 728 (Python 3.15) (#20009)

Add stubs for [PEP 728](https://peps.python.org/pep-0728/).

Author: cdce8p

mypy/test/teststubtest.py

@@ -88,6 +88,8 @@ class _TypedDict(Mapping[str, object]):
     __total__: ClassVar[bool]
     __readonly_keys__: ClassVar[frozenset[str]]
     __mutable_keys__: ClassVar[frozenset[str]]
+    __closed__: ClassVar[bool | None]
+    __extra_items__: ClassVar[Any]
 def overload(func: _T) -> _T: ...
 def type_check_only(func: _T) -> _T: ...
 def final(func: _T) -> _T: ...