[issue39889] Fix ast.unparse() for subscription by extended slices and tuples

[Serhiy Storchaka]

Change by Serhiy Storchaka <storchaka+cpython at gmail.com>:


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keywords: +patch
pull_requests: +18183
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/18824

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue39889>
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