gh-104050: Improve some typing around defaults and sentinel values

[AlexWaygood]

• 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.

• Issue: Add type annotations to clinic.py #104050

[AlexWaygood]

Not sure if converting the unspecified and unknown sentinels to become enum members is entirely worth it; now that default is typed as object anywhere, the benefits aren't so obvious. I do think it still makes the code a little cleaner, but you tell me whether you think it's worth the churn @erlend-aasland :)

[erlend-aasland]

Not sure if converting the unspecified and unknown sentinels to become enum members is entirely worth it; now that default is typed as object anywhere, the benefits aren't so obvious. I do think it still makes the code a little cleaner, but you tell me whether you think it's worth the churn @erlend-aasland :)

IMO, the more readable we can get clinic.py, the better. We should thread a little careful here, since test_clinic has pretty low coverage. Some of the paths modified are not tested. However, the CI does make clinic (for the check-generated-files job), and that would blow up if semantics changed for the code in question, so we're fine for now.

[AlexWaygood]

IMO, the more readable we can get clinic.py, the better. We should thread a little careful here, since test_clinic has pretty low coverage. Some of the paths modified are not tested. However, the CI does make clinic (for the check-generated-files job), and that would blow up if semantics changed for the code in question, so we're fine for now.

Fab. Thanks for the review!