[issue34822] Simplify AST for slices

[Serhiy Storchaka]

Serhiy Storchaka <storchaka+cpython at gmail.com> added the comment:

Updated the PR.

I can obviously be biased about my changes, so I need an approval of other core developer to merge them.

I created several PRs to popular third-party projects which work with AST to support both old and new AST schemes.

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versions: +Python 3.9 -Python 3.8

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