On Thursday, July 21, 2016 at 1:44:11 PM UTC-7, Guido van Rossum wrote:
I'm beginning to think that it would be useful to add
another ABC to collections.abc (and to typing) that represents the
intersection of these three ABCs, and to "upgrade" Set and Sequence to
inherit from it. (And Mapping.)

+1 on this, from the pytype side of things.
pytype, during type-inference, always struggles to give names to things, and having additional structural types around helps. 

Also, I like "Collection" better than Reiterable, since the latter sounds too intricate, possibly scaring users away.