Callable has no attribute __signature__

[kasium]

Bug Report

Mypy doesn't understand the __signature__ attribute of functions.
This was a part of #5958 and was now moved to it's own issue.

To Reproduce

import inspect

foo = lambda: None
foo.__signature__ = inspect.signature(foo)

Expected Behavior

Signature attribute exists

Actual Behavior

foo.py:4:1: error: "Callable[[], None]" has no attribute "__signature__" [attr-defined]

Your Environment

• Mypy version used: 0.942
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.7.1
• Operating system and version: Linux