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- updated to work with Tornado 5, which is now the minimum required version
- require pytest >= 3.0
- the `io_loop` fixture always refers to a `tornado.ioloop.IOLoop instance` now
- the `io_loop_asyncio` and `io_loop_tornado` fixtures have been removed, since
now that Tornado 5 always uses asyncio under Python 3, there would be no
difference between the two fixtures, so `io_loop` is all that is needed
- tox tests now test more versions of Tornado (5.0.* and latest 5.*),
Pytest (3.0.* and latest 3.*), and Python (3.5, 3.6, 3.7, and pypy3).