Inconsistent calls to __eq__ from built-in __contains__

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2016-07-24.12:13:15.023>
labels = ['type-bug', '3.8', '3.9', 'docs']
title = 'Inconsistent calls to __eq__ from built-in __contains__'
updated_at = <Date 2019-05-30.20:41:50.346>
user = 'https://bugs.python.org/vfaronov'

bugs.python.org fields:

activity = <Date 2019-05-30.20:41:50.346>
actor = 'cheryl.sabella'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation']
creation = <Date 2016-07-24.12:13:15.023>
creator = 'vfaronov'
dependencies = []
files = ['43857']
hgrepos = []
issue_num = 27605
keywords = []
message_count = 5.0
messages = ['271145', '271154', '271158', '271165', '271640']
nosy_count = 6.0
nosy_names = ['rhettinger', 'terry.reedy', 'steven.daprano', 'r.david.murray', 'docs@python', 'vfaronov']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue27605'
versions = ['Python 3.8', 'Python 3.9']