[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/timeouts.py

@@ -49,8 +49,9 @@ class Timeout:
 
     def reschedule(self, when: Optional[float]) -> None:
         """Reschedule the timeout."""
-        assert self._state is not _State.CREATED
         if self._state is not _State.ENTERED:
+            if self._state is _State.CREATED:
+                raise RuntimeError("Timeout has not been entered")
             raise RuntimeError(
                 f"Cannot change state of {self._state.value} Timeout",
             )
@@ -82,11 +83,14 @@ class Timeout:
         return f"<Timeout [{self._state.value}]{info_str}>"
 
     async def __aenter__(self) -> "Timeout":
-        self._state = _State.ENTERED
-        self._task = tasks.current_task()
-        self._cancelling = self._task.cancelling()
-        if self._task is None:
+        if self._state is not _State.CREATED:
+            raise RuntimeError("Timeout has already been entered")
+        task = tasks.current_task()
+        if task is None:
             raise RuntimeError("Timeout should be used inside a task")
+        self._state = _State.ENTERED
+        self._task = task
+        self._cancelling = self._task.cancelling()
         self.reschedule(self._when)
         return self