Support @no_type_check for classes

[JukkaL]

This are defined in the PEP 484 draft.

[JukkaL]

This is now supported for functions but not for classes.

[lunixbochs]

The title here is incorrect, the @no_type_check decorator is working for functions but @no_type_check_decorator does not appear to be implemented.

[gvanrossum]

Regarding the title, there seem to be two remaining issues.

• @no_type_check isn't implemented for classes
• @no_type_check_decorator isn't implemented at all

Note that the latter allows people to define their own decorators whose effect is equivalent to that of @no_type_check, i.e. this is a meta-level decorator.

AFAICT, #6584 will fix the first bullet but not the second. Perhaps we need to create a separate issue for the latter?

[gvanrossum]

OK, changed the title to reflect that this issue (#607) is about the @no_type_check decorator as applied to classes, while the new #6583 is about the "meta" decorator, @no_type_check_decorator.