forkserver could warn if several threads are running

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2017-10-31.18:12:26.725>
labels = ['3.7', 'type-feature', 'library']
title = 'forkserver could warn if several threads are running'
updated_at = <Date 2017-11-01.23:22:22.238>
user = 'https://github.com/pitrou'

bugs.python.org fields:

activity = <Date 2017-11-01.23:22:22.238>
actor = 'pitrou'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2017-10-31.18:12:26.725>
creator = 'pitrou'
dependencies = []
files = []
hgrepos = []
issue_num = 31913
keywords = []
message_count = 3.0
messages = ['305323', '305400', '305401']
nosy_count = 4.0
nosy_names = ['gregory.p.smith', 'pitrou', 'giampaolo.rodola', 'davin']
pr_nums = []
priority = 'low'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue31913'
versions = ['Python 3.7']