[issue44676] Add ability to serialise types.Union

[Ken Jin]

Change by Ken Jin <kenjin4096 at gmail.com>:


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nosy: +kj
nosy_count: 5.0 -> 6.0
pull_requests: +25878
pull_request: https://github.com/python/cpython/pull/27335

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue44676>
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