Tweaks to the TaskGroup API (#7255)

* Remove TaskGroup name arg and get_name() method

(We removed those from the implementation.)

* Add optional name arg to create_task()

See https://github.com/python/cpython/pull/31398

stdlib/asyncio/taskgroups.pyi

@@ -9,8 +9,7 @@ from .tasks import Task
 _T = TypeVar("_T")
 
 class TaskGroup:
-    def __init__(self, *, name: str | None = ...) -> None: ...
-    def get_name(self) -> str: ...
+    def __init__(self) -> None: ...
     async def __aenter__(self: 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]) -> Task[_T]: ...
+    def create_task(self, coro: Generator[Any, None, _T] | Coroutine[Any, Any, _T], *, name: str | None = ...) -> Task[_T]: ...