avoid try 1000 in asyncio all_tasks by making weak collection .copy() atomic

[graingert]

BPO	45276
Nosy	@asvetlov, @1st1, @graingert
PRs	bpo-45276: make weak collection's copy() atomic and use this feature to avoid race conditions in asyncio and threading #28541

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-09-24.08:10:36.669>
labels = ['interpreter-core', '3.11', '3.9', '3.10', 'expert-asyncio']
title = 'avoid try 1000 in asyncio all_tasks by making weak collection .copy() atomic'
updated_at = <Date 2021-09-24.08:17:16.047>
user = 'https://github.com/graingert'

bugs.python.org fields:

activity = <Date 2021-09-24.08:17:16.047>
actor = 'graingert'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Interpreter Core', 'asyncio']
creation = <Date 2021-09-24.08:10:36.669>
creator = 'graingert'
dependencies = []
files = []
hgrepos = []
issue_num = 45276
keywords = ['patch']
message_count = 1.0
messages = ['402544']
nosy_count = 3.0
nosy_names = ['asvetlov', 'yselivanov', 'graingert']
pr_nums = ['28541']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue45276'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

[graingert]

the weak collections should have the same threadsafe/thread unsafe guarantees as their strong reference counterparts - eg dict.copy and set.copy are atomic and so the weak versions should be atomic also

[kumaraditya303]

This is not how this should be fixed, relying on implicit implementation details should be avoided unless guaranteed. See #80788 (comment)