Callable type is missing the __signature__ attribute

[jrideout]

from typing import Callable

def test(f: Callable) -> None:
    f.__signature__ = ''

mypy returns:

error: "Callable[..., Any]" has no attribute "__signature__"

[jrideout]

I'm dealing with issues related to #270, but it seems regardless of what direction that goes __signature__ should be a recognized attribute of Callable

[ilevkivskyi]

This is not really a typing problem, but a mypy problem. Essentially, this is just a duplicate of python/mypy#5958