[ty] Fix accidental Liskov violation in protocol tests

[sharkdp]

Summary

We have the following test in protocols.md:

class HasX(Protocol):
    x: int

# […]

class Foo:
    x: int

# […]

class FooBool(Foo):
    x: bool

static_assert(not is_subtype_of(FooBool, HasX))
static_assert(not is_assignable_to(FooBool, HasX))

If Foo was indeed intended to be a base class of FooBool, then x: bool should be reported as a Liskov violation. And then it's a matter of definition whether or not these assertions should hold true or not (should the incorrect override take precedence or not?). So it looks to me like this is just an oversight, probably a copy-paste error from another test right before it, where FooSub is indeed intended to be a subclass of Foo.

I am fixing this because this test started to fail on a branch of mine that changes how attribute lookup in inheritance chains works.