Apply the None rule only to callable protocol members.

Currently mypy applies the rule that things like ``__iter__ = None`` block protocol implementation. However, it is applied only to dunder attributes, which is too ad-hoc (also there are names like ``send`` and ``throw`` in ``collections.abc``). It is proposed in https://github.com/python/typing/pull/464 that it should be applied only to methods (callable members) which would be more consistent.