Improve the signature of type.__subclasses__

[AlexWaygood]

ty currently infers the types of object.__subclasses__() as list[<class 'object'>], AKA, "a list in which all contents of the list are the class object". This is obviously suboptimal: object clearly has more subclasses than just itself.

All other type checkers infer the type of object.__subclasses__() as either list[type[object]] (pyright, pyrefly) or list[Callable[[], object]] (mypy), because no other type checker has the concept of "class-literal types" the same way that ty does. (A "class-literal" type like <class 'object'> means "exactly the class object, not including subclasses", unlike type[object] which means "the class object, or any subclass".) But ty's behaviour arguably isn't incorrect here: it's a valid solution to the type variable in typeshed's signature as it is right now.

With the changes made in this PR, mypy, ty, pyright and pyrefly all infer the type of int.__subclasses__() as list[type[int]].

Fixes astral-sh/ty#4386. Credit to @sharkdp for the idea of how to fix this.

[srittau]

Could we add a brief test here that this infers correctly for the type checkers supported by our tests?

[github-actions]

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