pythongh-88110: clear concurrent.futures.thread._threads_queues after…

… fork

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

    Traceback (most recent call last):
      File "/usr/lib64/python3.11/multiprocessing/popen_fork.py", line 72, in _launch
        code = process_obj._bootstrap(parent_sentinel=child_r)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      File "/usr/lib64/python3.11/multiprocessing/process.py", line 332, in _bootstrap
        threading._shutdown()
      File "/usr/lib64/python3.11/threading.py", line 1561, in _shutdown
        atexit_call()
      File "/usr/lib64/python3.11/concurrent/futures/thread.py", line 31, in _python_exit
        t.join()
      File "/usr/lib64/python3.11/threading.py", line 1109, in join
        raise RuntimeError("cannot join current thread")
    RuntimeError: cannot join current thread

Fixes python#88110

Lib/concurrent/futures/thread.py

@@ -41,6 +41,7 @@ def _python_exit():
     os.register_at_fork(before=_global_shutdown_lock.acquire,
                         after_in_child=_global_shutdown_lock._at_fork_reinit,
                         after_in_parent=_global_shutdown_lock.release)
+    os.register_at_fork(after_in_child=_threads_queues.clear)
 
 
 class _WorkItem(object):