Add missing methods to ItemsView and KeysView, including isdisjoint

[vr2262]

Fixes #2919

Unsure whether to write out the signatures of the various __r operator methods or do (for example)

__ror__ = __or__

which is what collection.abc.Set does. I opted to write them out explicitly.

Also unsure whether the change to isdisjoint is valid given the #TODO comment.

[srittau]

Thanks! I also prefer the explicit annotations of __ror__ et al, but it's mainly down to preference. the change to disjoint() looks good to me as well.

I am going to merge this, since the return values of the added methods match those of the existing ones, but I wonder why abstract return types where originally used when the views usually return quite concrete types.