Interrupting class creation in __init_subclass__ may lead to incorrect isinstance() and issubclass() results

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-09-10.09:09:41.066>
labels = ['interpreter-core', '3.8', 'type-bug', '3.7', '3.9']
title = 'Interrupting class creation in __init_subclass__ may lead to incorrect isinstance() and issubclass() results'
updated_at = <Date 2021-12-23.20:42:44.132>
user = 'https://bugs.python.org/xitop'

bugs.python.org fields:

activity = <Date 2021-12-23.20:42:44.132>
actor = 'Carl.Friedrich.Bolz'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2019-09-10.09:09:41.066>
creator = 'xitop'
dependencies = []
files = ['50515']
hgrepos = []
issue_num = 38085
keywords = ['patch']
message_count = 9.0
messages = ['351599', '355037', '358966', '381210', '383796', '401784', '401998', '409106', '409107']
nosy_count = 5.0
nosy_names = ['rhettinger', 'Carl.Friedrich.Bolz', 'lukasz.langa', 'hongweipeng', 'xitop']
pr_nums = ['30112']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue38085'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8', 'Python 3.9']