[3.12] gh-126500: test_ssl: Don't stop ThreadedEchoServer on OSError in ConnectionHandler; rely on __exit__ (GH-126503) (GH-126572)

gh-126500: test_ssl: Don't stop ThreadedEchoServer on OSError in ConnectionHandler; rely on __exit__ (GH-126503)

If `read()` in the ConnectionHandler thread raises `OSError` (except `ConnectionError`),
the ConnectionHandler shuts down the entire ThreadedEchoServer,
preventing further connections.
It also does that for `EPROTOTYPE` in `wrap_conn`.

As far as I can see, this is done to avoid the server thread getting stuck,
forgotten, in its accept loop. However, since 2011 (5b95eb90a7167285b6544b50865227c584943c9a)
the server is used as a context manager, and its `__exit__` does `stop()` and `join()`.
(I'm not sure if we *always* used `with` since that commit, but currently we do.)

Make sure that the context manager *is* used, and remove the `server.stop()`
calls from ConnectionHandler.
(cherry picked from commit c9cda1608edf7664c10f4f467e24591062c2fe62)

Co-authored-by: Petr Viktorin <encukou@gmail.com>