test_multiprocessing_fork hangs on an IPv6-only host

GitHub fields:

assignee = 'https://github.com/gpshead'
closed_at = None
created_at = <Date 2019-08-20.18:30:31.680>
labels = ['3.7', '3.8', 'type-bug', 'tests', '3.9']
title = 'test_multiprocessing_fork hangs on an IPv6-only host'
updated_at = <Date 2019-09-24.12:03:19.324>
user = 'https://github.com/gpshead'

bugs.python.org fields:

activity = <Date 2019-09-24.12:03:19.324>
actor = 'vstinner'
assignee = 'gregory.p.smith'
closed = False
closed_date = None
closer = None
components = ['Tests']
creation = <Date 2019-08-20.18:30:31.680>
creator = 'gregory.p.smith'
dependencies = []
files = []
hgrepos = []
issue_num = 37896
keywords = []
message_count = 1.0
messages = ['350032']
nosy_count = 2.0
nosy_names = ['gregory.p.smith', 'vstinner']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue37896'
versions = ['Python 3.7', 'Python 3.8', 'Python 3.9']