[3.11] gh-111085: Fix invalid state handling in TaskGroup and Timeout (GH-111111) (GH-111172)

asyncio.TaskGroup and asyncio.Timeout classes now raise proper RuntimeError
if they are improperly used.

* When they are used without entering the context manager.
* When they are used after finishing.
* When the context manager is entered more than once (simultaneously or
  sequentially).
* If there is no current task when entering the context manager.

They now remain in a consistent state after an exception is thrown,
so subsequent operations can be performed correctly (if they are allowed).

(cherry picked from commit 6c23635f2b)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: James Hilton-Balfe <gobot1234yt@gmail.com>

Lib/asyncio/taskgroups.py

@@ -54,16 +54,14 @@ class TaskGroup:
     async def __aenter__(self):
         if self._entered:
             raise RuntimeError(
-                f"TaskGroup {self!r} has been already entered")
-        self._entered = True
-
+                f"TaskGroup {self!r} has already been entered")
         if self._loop is None:
             self._loop = events.get_running_loop()
-
         self._parent_task = tasks.current_task(self._loop)
         if self._parent_task is None:
             raise RuntimeError(
                 f'TaskGroup {self!r} cannot determine the parent task')
+        self._entered = True
 
         return self