gh-41779: Allow defining the __dict__ and __weakref__ __slots__ for any class

[serhiy-storchaka]

• Issue: __slots__ for subclasses of variable length types #41779

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📚 Documentation preview 📚: https://cpython-previews--141755.org.readthedocs.build/

[encukou]

This looks good, thank you!

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot AMD64 Debian root 3.x (tier-1) has failed when building commit 4bcab46.

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/345/builds/12724) and take a look at the build logs.
4. Check if the failure is related to this commit (4bcab46) 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/345/builds/12724

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

==

Click to see traceback logs

Traceback (most recent call last):
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/test/support/__init__.py", line 847, in gc_collect
    gc.collect()
    ~~~~~~~~~~^^
ResourceWarning: unclosed file <_io.FileIO name=13 mode='wb' closefd=True>


Traceback (most recent call last):
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/test/support/__init__.py", line 847, in gc_collect
    gc.collect()
    ~~~~~~~~~~^^
ResourceWarning: unclosed file <_io.FileIO name=11 mode='wb' closefd=True>