Drop support for python 3.3, 3.5.[01] (#2116)

Drop support for python 3.3, 3.5.[01] (#2116)

* Drop support for python 3.3

In June 2017, Python 3.3 accounted for 0.2% of Tornado's downloads
from pypi.

* travis.yml: Use trusty, update pypy versions

Get a py35-compatible version of pypy3. Plain "pypy" no longer seems
to work in this environment, so specify a version.

Drop testing of python 2.7.8, which no longer seems to be available.

* gen,queues: Drop the "legacy aiter protocol"

This protocol changed in Python 3.5.2 (after being introduced in
3.5.0). Async iteration now requires that version of Python (or
newer).

Closes #2107

* travis.yml: Only run doctests on one version of python 3

* Get tests passing on latest version of pypy3