[issue42809] Improve pickle tests for recursive data

[Serhiy Storchaka]

New submission from Serhiy Storchaka <storchaka+cpython at gmail.com>:

The proposed PR extends tests for pickling recursive structures. The code is now more generic and covers more corner cases (found during work on issue36694).

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components: Tests
messages: 384220
nosy: alexandre.vassalotti, serhiy.storchaka
priority: normal
severity: normal
status: open
title: Improve pickle tests for recursive data
type: enhancement
versions: Python 3.10

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue42809>
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