I think jupyter_client will soon depend on tornado

[matthew-brett]

I believe I'm right in thinking that jupyter_client will need tornado as a pip dependency, because tornado is about to become an optional dependency for pyzmq:

zeromq/pyzmq#1121

[minrk]

Indeed. Overlooked here because notebook and ipykernel (the two primary consumers of jupyter-client) both rightly depend directly on tornado.

This isn't caught in the tests, which are running with prerelease pyzmq, because ipykernel is a test dependency, pulling in tornado.