gh-91126: Docs and tests for slotted dataclasses with __init_subclass__

[sobolevn]

• Issue: dataclass(slots=True) incompatible with __init_subclass__ #91126

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📚 Documentation preview 📚: https://cpython-previews--123342.org.readthedocs.build/

[sobolevn]

New look: https://cpython-previews--123342.org.readthedocs.build/en/123342/library/dataclasses.html

[carljm]

Thanks!

[carljm]

LGTM!

[sobolevn]

I will wait a bit for @ericvsmith's review and merge it in a couple of days. Thanks!

[miss-islington-app]

Thanks @sobolevn for the PR 🌮🎉.. I'm working now to backport this PR to: 3.12, 3.13.
🐍🍒⛏🤖

[bedevere-app]

GH-123568 is a backport of this pull request to the 3.13 branch.

[sobolevn]

One week later - merging :)

[bedevere-app]

GH-123569 is a backport of this pull request to the 3.12 branch.

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot PPC64LE RHEL8 Refleaks 3.13 has failed when building commit cfdf376.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/#/builders/1407/builds/136) and take a look at the build logs.
4. Check if the failure is related to this commit (cfdf376) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/#/builders/1407/builds/136

Failed subtests:

• test_create_connection_local_addr_skip_different_family - test.test_asyncio.test_events.EPollEventLoopTests.test_create_connection_local_addr_skip_different_family

Summary of the results of the build (if available):

Click to see traceback logs

Traceback (most recent call last):
  File "/home/buildbot/buildarea/3.13.cstratak-RHEL8-ppc64le.refleak/build/Lib/test/test_asyncio/test_events.py", line 694, in test_create_connection_local_addr_skip_different_family
    with self.assertRaises(OSError):
        self.loop.run_until_complete(f)
AssertionError: OSError not raised