Currently callable types in many error messages are formatted as

Callable[[Arg('x', int), VarArg(str)], int]

We should probably switch to more "native" format that is already used in some errors (e.g. for protocol and overloads):

def (x: int, *args: str) -> int

The main obstacle here is probably updating all tests that use the old format.

This is a follow up for #5463