why call _Py_set_inheritable(0) from os.open() when O_CLOEXEC?

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-04-12.14:46:33.267>
labels = ['3.8']
title = 'why call _Py_set_inheritable(0) from os.open() when O_CLOEXEC?'
updated_at = <Date 2019-04-12.14:50:18.302>
user = 'https://github.com/cagney'

bugs.python.org fields:

activity = <Date 2019-04-12.14:50:18.302>
actor = 'vstinner'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2019-04-12.14:46:33.267>
creator = 'cagney'
dependencies = []
files = []
hgrepos = []
issue_num = 36615
keywords = []
message_count = 2.0
messages = ['340050', '340051']
nosy_count = 2.0
nosy_names = ['vstinner', 'cagney']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue36615'
versions = ['Python 3.8']