Support @no_type_checks to ignore annotations for a function

[JukkaL]

Add a typing.no_type_checks function decorator that skips type checking a function and allows arbitrary annotations for the function. This should be give no errors:

from typing import no_type_checks

@no_type_checks
def foo(x: {'x': 2}, y: 1+2):
    1 + 'x'

See PEP 484 discussion: python/typing#26

[gvanrossum]

Update: it should be @no_type_check and @no_type_check_decorator as was chosen for the PEP and in some other discussion (can't find the issue right now).

[JukkaL]

This was implemented by @spkersten (thanks!).