> Yury's patch mostly looks good to me, except:

Thanks!

> * the check in contextlib should be against __cause__ rather than
__context__, and there should be a new test for this code handling path

Done.  I've also added one test for correct handling of StopIteration without PEP 479.

> * there should be a new test for the __future__ flag itself (independently
of the contextlib tests)

Forgot to attach it to the first patch!

Nick, please take a look at the new patch (attached).