Use typing_extensions.Self in the stdlib (#9694)

stdlib/asyncio/taskgroups.pyi

@@ -1,10 +1,10 @@
 # This only exists in 3.11+. See VERSIONS.
 
-from _typeshed import Self
 from collections.abc import Coroutine, Generator
 from contextvars import Context
 from types import TracebackType
 from typing import Any, TypeVar
+from typing_extensions import Self
 
 from .tasks import Task
 
@@ -13,7 +13,7 @@ __all__ = ["TaskGroup"]
 _T = TypeVar("_T")
 
 class TaskGroup:
-    async def __aenter__(self: Self) -> Self: ...
+    async def __aenter__(self) -> Self: ...
     async def __aexit__(self, et: type[BaseException] | None, exc: BaseException | None, tb: TracebackType | None) -> None: ...
     def create_task(
         self, coro: Generator[Any, None, _T] | Coroutine[Any, Any, _T], *, name: str | None = None, context: Context | None = None