handle tornado 6.1 on windows vs python 3.8

[bollwyvl]

The good news: tornado 6.1 obviates the need for the nasty event loop hack.

The bad news: if the event loop hack is applied, it causes intermittent hanging at some deep dark level, and needs to be killed with non-python fire.

I've sprinkled a hard removal of the hack in a bunch of jupyter_server stuff, but feel like we might need to be a bit more cautious here.

I'm thinking of just adding if tornado.version_info >= (6, 1): continue to all of the hacks, as that API has been stable back to our bottom pin.

Also, it's about time to get off the travis-ci bus. GitHub actions seems like the right play, we've been fairly successful with it across a number of other repos.

PR incoming, but it might take a while (i'll try to get it green on a personal PR first).

[bollwyvl]

Here's my WIP PR: bollwyvl#1

[bollwyvl]

I got green... going to squash'n'PR...