I reopen the issue: test_concurrent_futures seems to be also affected by this bug :-(

Example of recent hang:
https://github.com/python/cpython/runs/4908075699?check_suite_focus=true

---
0:18:04 load avg: 1.34 [417/418] test_peg_generator passed (9 min 46 sec) -- running: test_concurrent_futures (11 min 20 sec)
0:18:34 load avg: 0.81 running: test_concurrent_futures (11 min 50 sec)
0:19:04 load avg: 0.49 running: test_concurrent_futures (12 min 20 sec)
(...)
0:31:34 load avg: 0.00 running: test_concurrent_futures (24 min 50 sec)
Kill <TestWorkerProcess #3 running test=test_concurrent_futures pid=20092 time=25 min> process group
0:31:43 load avg: 0.00 [418/418/1] test_concurrent_futures timed out (25 min) (25 min)
(...)
0:31:43 load avg: 0.00 Re-running test_concurrent_futures in verbose mode
(...)

OK (skipped=6)

== Tests result: FAILURE then SUCCESS ==

409 tests OK.

10 slowest tests:
- test_concurrent_futures: 25 min
- test_peg_generator: 9 min 46 sec
- test_tools: 4 min 45 sec
- test_asyncio: 2 min 18 sec
- test_weakref: 1 min 26 sec
- test_venv: 1 min 24 sec
- test_subprocess: 1 min 6 sec
- test_lib2to3: 1 min 1 sec
- test_pickle: 59.4 sec
- test_gdb: 58.5 sec

9 tests skipped:
    test_devpoll test_ioctl test_kqueue test_msilib test_startfile
    test_winconsoleio test_winreg test_winsound test_zipfile64

1 re-run test:
    test_concurrent_futures
---