Bisecting led to this commit and reverting it on 3.8 branch makes the hang go away.

7c994549dcffd0d9d3bb37475e6374f356e7240e is the first bad commit
commit 7c994549dcffd0d9d3bb37475e6374f356e7240e
Author: Pablo Galindo <Pablogsal@gmail.com>
Date:   Sat Mar 16 22:34:24 2019 +0000

    bpo-35493: Use Process.sentinel instead of sleeping for polling worker status in multiprocessing.Pool (#11488)

    * bpo-35493: Use Process.sentinel instead of sleeping for polling worker status in multiprocessing.Pool

    * Use self-pipe pattern to avoid polling for changes

    * Refactor some variable names and add comments

    * Restore timeout and poll

    * Use reader object only on wait()

    * Recompute worker sentinels every time

    * Remove timeout and use change notifier

    * Refactor some methods to be overloaded by the ThreadPool, document the cache class and fix typos

 Lib/multiprocessing/pool.py                        | 88 +++++++++++++++++++---
 .../2019-01-09-23-43-08.bpo-35493.kEcRGE.rst       |  3 +
 2 files changed, 80 insertions(+), 11 deletions(-)
 create mode 100644 bpo-35493.kEcRGE.rst">Misc/NEWS.d/next/Library/2019-01-09-23-43-08.bpo-35493.kEcRGE.rst