attribute types

```py
class A:
a: Final[int]
b: TypeCheckOnly[str]
class A2(A):
a: Final[bool]

class B[T: A]:
c: T.a

b: B[A2]
reveal_type(b.c) # bool
```

Reproduce the Python snippet with basedmypy and inspect the type-checking path for generic attribute access and inherited attribute types. Done means the example is accepted with reveal_type(b.c) reported as bool, with regression coverage added for the demonstrated behavior.

Written by the indexing model from the issue text.