typing_extensions crash on Python 3.5.0 - 3.5.2 if used together with collections.abc

[ilevkivskyi]

Originally reported in dropbox/pyannotate#36

A simple repro

import typing_extensions
import collections.abc
isinstance(1, collections.abc.Mapping)

leads to a recursion error like this: https://travis-ci.org/ilevkivskyi/typehinting/jobs/306766341