Allow class variable as implementation for read only attribute

[ilevkivskyi]

Fixes #10289

Unless I am missing something, this indeed looks safe, so I am going to allow this.

cc @JukkaL

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[bzoracler]

If x: Foo = Bar() is allowed to pass with the example in #10289, then should Foo be allowed to override FooParent?

from typing import *

class FooParent(Protocol):
    foo: ClassVar[int]

class Foo(FooParent, Protocol):
    @property
    def foo(self) -> int:  # Signature of "foo" incompatible with supertype "FooParent" [override]
        ...

class Bar:
    foo: ClassVar[int]

x: Foo = Bar()  # Expected to pass
y: FooParent = Bar()  # Expected to pass (?)

[ilevkivskyi]

No, it should not. First, from Bar <: Foo and Bar <: FooParent one cannot conclude any subtyping between Foo vs FooParent. Second, it is actually unsafe, foo can be set freely on Type[FooParent], while on Type[Foo] it is at most read-only.

FWIW the opposite is true, FooParent should be allowed to override Foo (and it looks like it is allowed). So it looks like this PR actually fixes a hole in transitivity of subtyping.