test_concurrent_futures sometimes hangs indefinitely on Windows #92222

Bug report

test_concurrent_futures appears to be hanging indefinitely on Windows in our CI. This has happened twice three times in two days, on three separate unrelated CI runs.

Two PRs:

• gh-92032: Add soft keywords to rlcompleter #92029 (comment)
• bpo-45897: Fix frozen-slotted dataclass bug #29895: https://github.com/python/cpython/runs/6270908903?check_suite_focus=true

test_concurrent_futures has also been running for 4hr30 (and counting) on Windows (x86) on this recent push to main:

• 9b027d4: https://github.com/python/cpython/runs/6269422000?check_suite_focus=true

Neither PR had anything to do with concurrent.futures (and the push to main didn't have anything to do with concurrent.futures either). For the first PR, test_concurrent_futures was hanging on Windows (x64) but passed on Windows (x86). For the second PR, the test has passed on Windows (x64) but is hanging on Windows (x86).

cc. @pitrou, @brianquinlan, as listed experts for concurrent.futures.