Use the runtime's main thread ID in the threading module.

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-12-13.22:21:22.139>
labels = ['3.8', 'type-bug', 'library', '3.9']
title = "Use the runtime's main thread ID in the threading module."
updated_at = <Date 2020-02-25.12:06:41.723>
user = 'https://github.com/ericsnowcurrently'

bugs.python.org fields:

activity = <Date 2020-02-25.12:06:41.723>
actor = 'lukasz.langa'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2019-12-13.22:21:22.139>
creator = 'eric.snow'
dependencies = []
files = []
hgrepos = []
issue_num = 39042
keywords = ['3.8regression']
message_count = 4.0
messages = ['358362', '358557', '358727', '362634']
nosy_count = 2.0
nosy_names = ['lukasz.langa', 'eric.snow']
pr_nums = []
priority = 'high'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue39042'
versions = ['Python 3.8', 'Python 3.9']