[issue46242] Improve error message when attempting to extend an enum with `__call__`

[Alex Waygood]

Change by Alex Waygood <Alex.Waygood at Gmail.com>:


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resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue46242>
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