IO streams locking can be broken after fork() with threads

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-04-26.16:58:22.058>
labels = ['3.8', 'type-bug', 'library', '3.9']
title = 'IO streams locking can be broken after fork() with threads'
updated_at = <Date 2020-04-29.16:43:20.805>
user = 'https://github.com/Delgan'

bugs.python.org fields:

activity = <Date 2020-04-29.16:43:20.805>
actor = 'pitrou'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2020-04-26.16:58:22.058>
creator = 'Delgan'
dependencies = []
files = ['49094']
hgrepos = []
issue_num = 40399
keywords = []
message_count = 8.0
messages = ['367317', '367509', '367510', '367516', '367522', '367525', '367674', '367675']
nosy_count = 2.0
nosy_names = ['pitrou', 'Delgan']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue40399'
versions = ['Python 3.8', 'Python 3.9']