[ty] Revert protocol ClassVar declaration requirement

[AlexWaygood]

Reverts #27530.

Restore the previous protocol-matching behavior so any type T can satisfy a protocol with a ClassVar member x if T is readable on instances of T, and both readable and writable on type[T].

Cf. python/typing#2340

[astral-sh-bot]

Typing conformance results regressed ❌

The percentage of diagnostics emitted that were expected errors decreased from 97.14% to 97.14%. The percentage of expected errors that received a diagnostic decreased from 93.44% to 93.43%. The number of fully passing files held steady at 107/137.

Summary

How are test cases classified?

Each test case represents one expected error annotation or a group of annotations sharing a tag. Counts are per test case, not per diagnostic — multiple diagnostics on the same line count as one. Required annotations (E) are true positives when ty flags the expected location and false negatives when it does not. Optional annotations (E?) are true positives when flagged but true negatives (not false negatives) when not. Tagged annotations (E[tag]) require ty to flag exactly one of the tagged lines; tagged multi-annotations (E[tag+]) allow any number up to the tag count. Flagging unexpected locations counts as a false positive.

Metric	Old	New	Diff	Outcome
True Positives	1054	1052	-2	⏬ (❌)
False Positives	31	31	+0
False Negatives	74	74	+0
Total Diagnostics	1127	1125	-2	⏬
Precision	97.14%	97.14%	-0.01%	⏬ (❌)
Recall	93.44%	93.43%	-0.01%	⏬ (❌)
Passing Files	107/137	107/137	+0

Test file breakdown

2 files altered

File	True Positives	False Positives	False Negatives	Status
protocols_class_objects.py	4 (-1) ❌	0	3	➡️ Neutral
protocols_definition.py	22 (-1) ❌	0	0	✅ Still Passing
Total (all files)	1052 (-2) ❌	31	74	107/137

Optional Diagnostics Removed (2)

2 diagnostics

Test case	Diff
protocols_class_objects.py:108	-error[invalid-assignment] Object of type `<class 'ConcreteC3'>` is not assignable to `ProtoC1`
protocols_definition.py:117	-error[invalid-assignment] Object of type `Concrete2_Bad4` is not assignable to `Template2`

[AlexWaygood]

Bah, we gotta fix our typing-conformance script so it doesn't display those demotivating ❌ emojis if the only diagnostics removed are optional diagnostics 😆

[astral-sh-bot]

Memory usage report

Memory usage unchanged ✅

[AlexWaygood]

Ummmmm, I have no idea why there's no ecosystem-analyzer comment here, but the uploaded report at https://github.com/astral-sh/ruff/actions/runs/31708813786?pr=27726 says that there's no ecosystem changes

[charliermarsh]

Can you explain whether this is still a TODO? (And, if so, why revert?)