test_deadlock sometimes causes deadlock

Bug report

Bug description:

Found at EuroPython 24 sprints.

./python -m test -j4

sometimes ends up hanging on

0:24:44 load avg: 0.80 running (1): test.test_concurrent_futures.test_deadlock (24 min 3 sec)
0:25:14 load avg: 0.56 running (1): test.test_concurrent_futures.test_deadlock (24 min 33 sec)
0:25:44 load avg: 0.47 running (1): test.test_concurrent_futures.test_deadlock (25 min 3 sec)
0:26:14 load avg: 0.35 running (1): test.test_concurrent_futures.test_deadlock (25 min 33 sec)
0:26:44 load avg: 0.45 running (1): test.test_concurrent_futures.test_deadlock (26 min 3 sec)
0:27:14 load avg: 0.54 running (1): test.test_concurrent_futures.test_deadlock (26 min 33 sec)

Also occurs on @stinovlas machine (Debian or Ubuntu). More details when I'll be reproduce deadlock again.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

首先运行 ./python -m test -j4，并在 Linux 上的 CPython main 中单独运行 test.test_concurrent_futures.test_deadlock。阅读该测试及其周围的 concurrent.futures 代码，然后可靠地复现间歇性挂起。完成的标准是测试套件不再卡在此处，并且相关测试覆盖了这次回归。