[Enum] _sunder_ methods only looked up in the last Enum class in the mro

GitHub fields:

assignee = 'https://github.com/ethanfurman'
closed_at = None
created_at = <Date 2020-12-18.20:07:22.590>
labels = ['type-bug', 'library', '3.9', '3.10']
title = '[Enum] _sunder_ methods only looked up in the last Enum class in the mro'
updated_at = <Date 2020-12-18.20:07:22.590>
user = 'https://github.com/ethanfurman'

bugs.python.org fields:

activity = <Date 2020-12-18.20:07:22.590>
actor = 'ethan.furman'
assignee = 'ethan.furman'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2020-12-18.20:07:22.590>
creator = 'ethan.furman'
dependencies = []
files = []
hgrepos = []
issue_num = 42678
keywords = []
message_count = None
messages = []
nosy_count = 1.0
nosy_names = ['ethan.furman']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue42678'
versions = ['Python 3.9', 'Python 3.10']