Migrate sentinels to use Python 3.15's PEP 661 `sentinel` built-in with typing_extensions backport
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@AndreasBackx is already working on this.
Since May 4, 2026.
typing
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Description
Migrate sentinel values like at src/click/_utils.py to use the introduced sentinel built-in coming in Python 3.15. For older versions we can use typing_extenions.sentinel.
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