getting a class source regresses in Python 3.9

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-12-17.09:06:46.854>
labels = ['type-bug', 'library', '3.9', '3.10']
title = 'getting a class source regresses in Python 3.9'
updated_at = <Date 2020-12-17.09:11:37.904>
user = 'https://github.com/t-vi'

bugs.python.org fields:

activity = <Date 2020-12-17.09:11:37.904>
actor = 't-vi'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2020-12-17.09:06:46.854>
creator = 't-vi'
dependencies = []
files = []
hgrepos = []
issue_num = 42666
keywords = []
message_count = 2.0
messages = ['383223', '383224']
nosy_count = 1.0
nosy_names = ['t-vi']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue42666'
versions = ['Python 3.9', 'Python 3.10']