[3.12] gh-88110: Clear concurrent.futures.thread._threads_queues after fork to avoid...

[3.12] gh-88110: Clear concurrent.futures.thread._threads_queues after fork to avoid joining parent process' threads (GH-126098) (GH-127164)

Threads are gone after fork, so clear the queues too. Otherwise the
child process (here created via multiprocessing.Process) crashes on
interpreter exit.

(cherry picked from commit 1848ce61f349533ae5892a8c24c2e0e3c364fc8a)

Co-authored-by: Andrei Bodrov <Drino@users.noreply.github.com>
Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>