"signal only works in main thread" in main thread

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-11-24.06:06:20.843>
labels = ['type-bug', '3.8']
title = '"signal only works in main thread" in main thread'
updated_at = <Date 2019-12-17.17:14:27.640>
user = 'https://bugs.python.org/RichardWarfield'

bugs.python.org fields:

activity = <Date 2019-12-17.17:14:27.640>
actor = 'Richard Warfield'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2019-11-24.06:06:20.843>
creator = 'Richard Warfield'
dependencies = []
files = []
hgrepos = []
issue_num = 38904
keywords = []
message_count = 6.0
messages = ['357390', '357391', '358363', '358466', '358558', '358559']
nosy_count = 3.0
nosy_names = ['vstinner', 'eric.snow', 'Richard Warfield']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue38904'
versions = ['Python 3.8']