"Impossible" KeyError from importlib._bootstrap acquire line 110

GitHub fields:

assignee = None
closed_at = <Date 2021-09-11.10:54:03.261>
created_at = <Date 2021-03-18.18:58:26.236>
labels = ['3.9', 'type-crash']
title = '"Impossible" KeyError from importlib._bootstrap acquire line 110'
updated_at = <Date 2021-09-11.10:54:03.261>
user = 'https://github.com/anentropic'

bugs.python.org fields:

activity = <Date 2021-09-11.10:54:03.261>
actor = 'iritkatriel'
assignee = 'none'
closed = True
closed_date = <Date 2021-09-11.10:54:03.261>
closer = 'iritkatriel'
components = []
creation = <Date 2021-03-18.18:58:26.236>
creator = 'anentropic'
dependencies = []
files = []
hgrepos = []
issue_num = 43546
keywords = []
message_count = 7.0
messages = ['389034', '389035', '389036', '389037', '389038', '389067', '389085']
nosy_count = 6.0
nosy_names = ['brett.cannon', 'ncoghlan', 'pitrou', 'eric.snow', 'serhiy.storchaka', 'anentropic']
pr_nums = []
priority = 'normal'
resolution = 'third party'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'crash'
url = 'https://bugs.python.org/issue43546'
versions = ['Python 3.9']