deepcopy of weakref proxies

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-02-18.12:19:01.973>
labels = ['type-bug', 'library', '3.9']
title = 'deepcopy of weakref proxies'
updated_at = <Date 2021-04-15.08:26:12.123>
user = 'https://github.com/konrad-schwarz'

bugs.python.org fields:

activity = <Date 2021-04-15.08:26:12.123>
actor = 'konrad.schwarz'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2021-02-18.12:19:01.973>
creator = 'konrad.schwarz'
dependencies = []
files = []
hgrepos = []
issue_num = 43252
keywords = []
message_count = 3.0
messages = ['387226', '391094', '391121']
nosy_count = 2.0
nosy_names = ['iritkatriel', 'konrad.schwarz']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue43252'
versions = ['Python 3.9']