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

Start by running ./python -m test -j4 and isolate test.test_concurrent_futures.test_deadlock on CPython main under Linux. Read the test and its surrounding concurrent.futures code, then reproduce the intermittent hang reliably. Done means the test suite no longer remains stuck there and the regression is covered by the relevant test.