Cryptic error message when creating types that don't include themselves in their MRO

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-02-17.17:00:26.647>
labels = ['interpreter-core', '3.8', 'type-bug', '3.7', '3.9']
title = "Cryptic error message when creating types that don't include themselves in their MRO"
updated_at = <Date 2020-03-26.00:03:35.085>
user = 'https://github.com/pppery'

bugs.python.org fields:

activity = <Date 2020-03-26.00:03:35.085>
actor = 'ppperry'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2020-02-17.17:00:26.647>
creator = 'ppperry'
dependencies = []
files = []
hgrepos = []
issue_num = 39665
keywords = []
message_count = 2.0
messages = ['362152', '362154']
nosy_count = 2.0
nosy_names = ['serhiy.storchaka', 'ppperry']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue39665'
versions = ['Python 3.7', 'Python 3.8', 'Python 3.9']