type variables should be allowed on `ClassVar`s

[KotlinIsland]

class A[T]:
    t: ClassVar[T]
A[int].t = 1

this is obviously an error, but:

class B(A[int]):
    pass
B.t = 1

this should be allowed

[KotlinIsland]

also this

class A[T]:
    class B[TB = T]: ...
class C(A[int]: ...

A.B() # error
A[int].B() # ok
C.B() # ok

[KotlinIsland]

the issue is with classmethod:

class A:
    a: ClassVar[T]
    @classvar
    def f(cls, t: T):
        cls.a = t
A[int].f(1)

A[int].f is stupid

but this is often how constructors are written:

    @classmethod
    def make(cls, t: T) -> Self:
        return cls(t)

so the question is: how can we support both cases safely