[issue38250] enum.Flag should be more set-like

[Ethan Furman]

Change by Ethan Furman <ethan at stoneleaf.us>:


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pull_requests: +23042
pull_request: https://github.com/python/cpython/pull/24215

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue38250>
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