support abstract attributes/class attributes

```py
class A(ABC):
a: Abstract[ClassVar[str]]
b: Abstract[Final[str]]
```

No file, test, or entry point is named. Start by locating the existing handling and tests for Abstract, ClassVar, and Final in the type checker, then determine how the two annotated attributes should be validated. Done means the examples in the issue are supported with appropriate static-type-checking behavior and regression coverage.

Written by the indexing model from the issue text.