gh-104050: Improve some typing around `default`s and sentinel values (#104626)

- Convert `unspecified` and `unknown` to be members of a `Sentinels` enum, rather than instances of bespoke classes.
  - An enum feels more idiomatic here, and works better with type checkers.
  - Convert some `==` and `!=` checks for these values to identity checks, which are more idiomatic with sentinels.
  - _Don't_ do the same for `Null`, as this needs to be a distinct type due to its usage in `clinic.py`.
- Use `object` as the annotation for `default` across `clinic.py`. `default` can be literally any object, so `object` is the correct annotation here.

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Co-authored-by: Erlend E. Aasland <erlend.aasland@protonmail.com>

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