python/qmp: remove Server.wait_closed() call for Python 3.12

python/qmp: remove Server.wait_closed() call for Python 3.12

This patch is a backport from
https://gitlab.com/qemu-project/python-qemu-qmp/-/commit/e03a3334b6a477beb09b293708632f2c06fe9f61

According to Guido in https://github.com/python/cpython/issues/104344 ,
this call was never meant to wait for the server to shut down - that is
handled synchronously - but instead, this waits for all connections to
close. Or, it would have, if it wasn't broken since it was introduced.

3.12 fixes the bug, which now causes a hang in our code. The fix is just
to remove the wait.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Message-id: 20231006195243.3131140-3-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>
(cherry picked from commit acf873873ae38e68371b0c53c42d3530636ff94e)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>