multiprocessing: passing file descriptor using reduction breaks duplex pipes on darwin

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2017-12-05.08:56:06.683>
labels = ['OS-mac', 'type-bug', 'library', '3.7']
title = 'multiprocessing: passing file descriptor using reduction breaks duplex pipes on darwin'
updated_at = <Date 2017-12-06.12:06:38.226>
user = 'https://bugs.python.org/frickenate'

bugs.python.org fields:

activity = <Date 2017-12-06.12:06:38.226>
actor = 'ronaldoussoren'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)', 'macOS']
creation = <Date 2017-12-05.08:56:06.683>
creator = 'frickenate'
dependencies = []
files = []
hgrepos = []
issue_num = 32220
keywords = []
message_count = 5.0
messages = ['307649', '307650', '307700', '307714', '307733']
nosy_count = 5.0
nosy_names = ['ronaldoussoren', 'pitrou', 'ned.deily', 'davin', 'frickenate']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue32220'
versions = ['Python 3.6', 'Python 3.7']