[3.12] gh-111085: Fix invalid state handling in TaskGroup and Timeout (GH-111111...

[3.12] gh-111085: Fix invalid state handling in TaskGroup and Timeout (GH-111111) (GH-111171)

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 6c23635f2b7067ef091a550954e09f8b7c329e3f)

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