gh-105829: Fix concurrent.futures.ProcessPoolExecutor deadlock (#108513)

This fixes issue #105829, https://github.com/python/cpython/issues/105829

Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>
Co-authored-by: Antoine Pitrou <antoine@python.org>
Co-authored-by: Chris Withers <chris@withers.org>
Co-authored-by: Thomas Moreau <thomas.moreau.2010@gmail.com>