[issue43945] [Enum] standardize format() behavior

[Ethan Furman]

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


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pull_requests: +25337
pull_request: https://github.com/python/cpython/pull/26752

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue43945>
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