handle stringified types consistently

[KotlinIsland]

# python3.8
a: "list[int]"  # error: "list" is not subscriptable, use "typing.List" instead
cast("list[int]", [])  # valid
cast(List["list[int]"], [])  # error

cast has inconsistent behaviour because I think it makes sense to allow it like that, so why not all stringified types?

maybe because of introspection reasons? which don't apply to cast.