unpickling a subclass of list fails when it implements its own extend method

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-04-27.00:33:02.281>
labels = ['3.8', '3.9', 'extension-modules', '3.7', 'type-bug', '3.10', '3.11', 'library']
title = 'unpickling a subclass of list fails when it implements its own extend method'
updated_at = <Date 2021-04-27.16:05:29.529>
user = 'https://github.com/gpshead'

bugs.python.org fields:

activity = <Date 2021-04-27.16:05:29.529>
actor = 'gregory.p.smith'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Extension Modules', 'Library (Lib)']
creation = <Date 2021-04-27.00:33:02.281>
creator = 'gregory.p.smith'
dependencies = []
files = []
hgrepos = []
issue_num = 43946
keywords = ['3.7regression']
message_count = 4.0
messages = ['392008', '392022', '392027', '392100']
nosy_count = 3.0
nosy_names = ['gregory.p.smith', 'serhiy.storchaka', 'richardlev']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue43946'
versions = ['Python 3.7', 'Python 3.8', 'Python 3.9', 'Python 3.10', 'Python 3.11']