Allow non-concrete TypeVar bounds

[ndmitchell]

Describe the Bug

from typing import *

ArgumentT = TypeVar("ArgumentT", bound=slice) # E: Type variable bounds and constraints must be concrete

This works fine in Pyright and Mypy.

Sandbox Link

https://pyrefly.org/sandbox/?code=GYJw9gtgBALgngBwJYDsDmUkQWEMoBUAUEQIIhoCuEApijACpQC8UDiNAagIYgAUAInJVa9BgIA0UAEZhKKACbMAzgBskAYxoBKIkA&version=3.12

(Only applicable for extension issues) IDE Information

No response

[samwgoldman]

Smaller repro -- the issue is that slice's type vars have defaults pointing to other type vars. We figure this out properly in other scenarios, but not in typevar bounds for some reason.

class C[A = int, B = A]:
    pass

def f[T: C]():
    pass

[rchen152]

This was fixed by 44457a9.