Bump Python, jlab, and tornado versions.

[TomAugspurger]

• python 3.6 -> 3.7
• jupyterlab -> 1.0.0
• tornado 5.1.1 -> 6.0.3

[TomAugspurger]

Are we OK with the 3.6 -> 3.7 change? Or should I just focus on jupyterlab for now?

[scottyhq]

This is great, thanks @TomAugspurger! Are the Tornado>6 issues fixed? Seemed like there were lots of issues with JupyterHub and dask previously. We tried bumping to Python 3.7 a while back and there was some issue building the images... Maybe with repo2docker, but I think it should be fine now.

[TomAugspurger]

Distributed fixed the tornado 6 issues in dask/distributed#2570. Notebook fixed them in jupyter/notebook#4439. So we should be OK, but who knows :)

I'm not sure when to deploy updates to binder.pangeo.io and prod.pangeo.io. I'll be offline tomorrow and most of Friday for the holiday.

[scottyhq]

going to go ahead and merge this to experiment w/ jupyterlab 1.0. i'll just note that things currently seem to work well with Docker image 2019.07.01 if things break.