Fix type of Future._exception (#10502)

stdlib/asyncio/futures.pyi

@@ -31,7 +31,7 @@ def isfuture(obj: object) -> TypeGuard[Future[Any]]: ...
 class Future(Awaitable[_T], Iterable[_T]):
     _state: str
     @property
-    def _exception(self) -> BaseException: ...
+    def _exception(self) -> BaseException | None: ...
     _blocking: bool
     @property
     def _log_traceback(self) -> bool: ...