FIx stubtest's tests to work with the latest version of typing_extensions (python#16928)

Stubtest's tests will start failing when `typing_extensions==4.10.0`
comes out, due to some new `ClassVar`s on `typing_extensions.TypedDict`.
This PR fixes that.

Fixes python/typing_extensions#339. Note:
there's no need to cherry-pick this to the `release-1.9.0` branch, since
the daily workflow `typing_extensions` uses runs mypy's tests using the
mypy `master` branch.

Author: AlexWaygood

test-data/unit/lib-stub/typing_extensions.pyi

@@ -61,6 +61,8 @@ class _TypedDict(Mapping[str, object]):
     __optional_keys__: frozenset[str]
     __readonly_keys__: frozenset[str]
     __mutable_keys__: frozenset[str]
+    __closed__: bool
+    __extra_items__: Any
     __total__: bool
 
 def TypedDict(typename: str, fields: Dict[str, Type[_T]], *, total: Any = ...) -> Type[dict]: ...