Check for protocol methods returning None

[srittau]

This is actually not a stub-specific rule, but protocol methods returning None instead of object are suspect. None is meant as "The caller should not use the return value", but it actually means that any implementation must return None. (I remember we had this problem at least once. It might have been related to an ignored seek() return value.)

[Akuli]

We should probably check for Callable[something, None] as well.