Document that collections.abc types are subscriptable

[FFY00]

BPO	45449
Nosy	@gvanrossum, @ambv, @miss-islington, @FFY00, @Fidget-Spinner
PRs	bpo-45449: add note about PEP 585 in collections.abc's documentation #29047 [3.10] bpo-45449: add note about PEP 585 in collections.abc's documentation (GH-29047) #29067 [3.9] bpo-45449: add note about PEP 585 in collections.abc's documentation (GH-29047) #29068

^{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 = <Date 2021-10-19.18:50:43.564>
created_at = <Date 2021-10-12.17:11:41.595>
labels = ['3.11', 'type-feature', '3.9', '3.10', 'docs']
title = 'Document that collections.abc types are subscriptable'
updated_at = <Date 2021-10-19.18:50:43.563>
user = 'https://github.com/FFY00'

bugs.python.org fields:

activity = <Date 2021-10-19.18:50:43.563>
actor = 'lukasz.langa'
assignee = 'docs@python'
closed = True
closed_date = <Date 2021-10-19.18:50:43.564>
closer = 'lukasz.langa'
components = ['Documentation']
creation = <Date 2021-10-12.17:11:41.595>
creator = 'FFY00'
dependencies = []
files = []
hgrepos = []
issue_num = 45449
keywords = ['patch']
message_count = 6.0
messages = ['403744', '403746', '404323', '404328', '404333', '404334']
nosy_count = 6.0
nosy_names = ['gvanrossum', 'docs@python', 'lukasz.langa', 'miss-islington', 'FFY00', 'kj']
pr_nums = ['29047', '29067', '29068']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue45449'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

[FFY00]

Since 3.9, the types in collections.abc are subscriptable and can be used as type hints, but this is not documented.
We should write something similar to what's on the typing module documentation for the deprecated types.

[FFY00]

Just to clarify: some type checkers will still allow using them as type hints in Python versions before 3.9, but from __future__ import annotations needs to be used so that we don't error out at ruuntime.

[ambv]

New changeset 7bafa0c by Filipe Laíns in branch 'main':
bpo-45449: add note about PEP-585 in collections.abc's documentation (GH-29047)
7bafa0c

[ambv]

New changeset 092ec4b by Miss Islington (bot) in branch '3.10':
bpo-45449: add note about PEP-585 in collections.abc's documentation (GH-29047) (GH-29067)
092ec4b

[ambv]

New changeset a18e4e9 by Łukasz Langa in branch '3.9':
[3.9] bpo-45449: add note about PEP-585 in collections.abc's documentation (GH-29047) (GH-29068)
a18e4e9

[ambv]

Thanks, Filipe! ✨ 🍰 ✨