Use the PEP 661 sentinel type where appropriate

[agronholm]

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Changes

This changes our current sentinels to use the PEP 661 sentinel type instead of an anonymous object.

The only place where this could not be done is the typed_attribute() function, as it is typed to return Any.

This PR needs a newer mypy for type checks to pass, as it doesn't yet recognize typing_extensions.sentinel.

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- Fix big bad boo-boo in task groups
  (`#123 <https://github.com/agronholm/anyio/issues/123>`_; PR by @yourgithubaccount)

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[tapetersen]

I know I saw at least one place a 1 member enum was used instead just for the debuggability reason. I'll see if I can find it again.

[tapetersen]

Found it (and it seems like the only one).

anyio/src/anyio/lowlevel.py

Lines 102 to 103 in b1f6e40

	class _NoValueSet(enum.Enum):
	NO_VALUE_SET = enum.auto()

[tapetersen]

Ok it uses it to have typing support in the code as well and until that is implemented it's probably not worth trying to change it.

I tried replacing it but mypy hasn't implemented it and with the flag to enable it in pyright they don't support the final semantics it seems yet (Sentinel vs., sentinel)

[agronholm]

Ok it uses it to have typing support in the code as well and until that is implemented it's probably not worth trying to change it.

I tried replacing it but mypy hasn't implemented it and with the flag to enable it in pyright they don't support the final semantics it seems yet (Sentinel vs., sentinel)

Tracking issue here.