Modernize the Server

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• Switch to Python 3.5+ so we can use async/await
• Use async/await instead of gen.coroutine
• Switch to pytest
• Remove ipython_genutils
• Some PEP8 changes along the way

To view the diff without whitespace changes: https://github.com/jupyter/jupyter_server/pull/23/files?w=1

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@minrk, of note, see the use of force_async in place of gen.maybe_future, which is deprecated and didn't get along with async/await.

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And of course, 12/26 commits come from CI problems...

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Annoyingly, we had to wrap every use of yield with force_async, because unlike modern JS, Python won't consume a non-awaitable object. This works in JS: let foo = await 1; console.log(foo), who knew it would start being better than Python 😉?

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Submitted an issue to the python bug tracker: https://bugs.python.org/issue34380

[minrk]

Raising when you try to await something you can't await seems like a feature, not a bug to me ;)

This is really great! I think we need to think pretty carefully about the transition, as this repo needs to be very proactively porting every PR that lands in jupyter/notebook in order to avoid divergence problems. Modernizing the code like this is super nice, but it will also make that transition period a whole lot harder by guaranteeing that ~all patches need manual changes in order to apply cleanly.

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Solid points Min, let's close this...