[issue44654] Refactor and clean up the union type implementation

[Serhiy Storchaka]

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


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pull_requests: +25764
pull_request: https://github.com/python/cpython/pull/27223

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue44654>
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