Add support for Python 3.6.

[tseaver]

I don't think Travis or Appveyor support 3.6 yet.

[dhermes]

LGTM

[dhermes]

@jonparrott @tseaver Have you heard of anyone dropping support for 3.4 / not testing 3.4 in CI any longer?

Already after adding 3.5 our "full" builds time out half the time on Travis

[tseaver]

@dhermes I'm contemplating trying to speed the build up by dropping our hacked-around namespace stuff: a quick test seems to indicate that pip install -e now works.

[dhermes]

@tseaver I welcome the change. That'll just require lower bounds on pip / setuptools in our CircleCI / Travis / AppVeyor configs?

[lukesneeringer]

Travis and CircleCI both have Python 3.6 support now. :-)

[theacodes]

Ya'll will probably need to wait on grpc/grpc#9445 just like google-auth googleapis/google-auth-library-python#102

[lukesneeringer]

This is available now, but I am actually closing this because I am working on a more dramatic overhaul of our testing tooling.