Allow setting __classcell__

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2022-03-28.14:32:05.171>
labels = ['interpreter-core', '3.11']
title = 'Allow setting __classcell__'
updated_at = <Date 2022-03-28.16:28:10.373>
user = 'https://github.com/douglas-raillard-arm'

bugs.python.org fields:

activity = <Date 2022-03-28.16:28:10.373>
actor = 'douglas-raillard-arm'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2022-03-28.14:32:05.171>
creator = 'douglas-raillard-arm'
dependencies = []
files = []
hgrepos = []
issue_num = 47144
keywords = []
message_count = 4.0
messages = ['416172', '416173', '416187', '416188']
nosy_count = 2.0
nosy_names = ['vstinner', 'douglas-raillard-arm']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue47144'
versions = ['Python 3.11']