gh-118761: Use enum._simple_enum for annotationlib.Format

[AA-Turner]

We can use the internal _simple_enum class decorator here, for slight faster enum creation. I don't think there's any good way to entirely avoid/defer the enum import, though.

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• Issue: Improve import time of various stdlib modules #118761

[JelleZijlstra]

I'm not a fan of using private functionality here; I'd rather this enum work the same way as an equivalent user-created way. If we think this is the right way to make an enum, we should make this decorator public.

[AA-Turner]

My understanding is it's an intentionally internal-only decorator (eg used in ast and re). @ethanfurman is probably best placed to comment.

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[python-cla-bot]

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