bpo-38967: Improve the error msg for reserved _sunder_ names in enum

[ZackerySpytz]

https://bugs.python.org/issue38967

Automerge-Triggered-By: @ethanfurman

[ZackerySpytz]

IMO, this doesn't need a news entry.

[ethanfurman]

Status checks are stuck; closing and reopening to try to get them to run again.

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