[issue42567] Enum: manually call __init_subclass__ after members are added

[Ethan Furman]

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


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pull_requests: +22628
stage: backport needed -> patch review
pull_request: https://github.com/python/cpython/pull/23772

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue42567>
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