add github actions, bail on asyncio patch for tornado 6.1

[bollwyvl]

This PR:

• removes appveyor and travis
• adds github actions
  • tests (py3.{6,7,8,9} + pypy) x {win,lin,mac}
    • couldn't even really get 3.5 going, not sure what's up
  • codecov
  • wheel caching
• adds skips for some osx/pypy tests that just aren't working
  • we already skip them in conda-forge downstream
• adds a version check for the tornado asyncio patch(es) on windows

Almost any of the excursions can flake, but at least they flake fast and hard instead of locking up until the CI reaps them for timeout.

Links:

• fixes handle tornado 6.1 on windows vs python 3.8 #563
• test pr with lots of fiddly commits: add github actions, bail on asyncio patch for tornado 6.1 bollwyvl/ipykernel#1

[blink1073]

Looks good, thank you!

[Zsailer]

This might warrant a quick patch release here. Things are a bit flaky in jupyter_server's CI and I think it's related to this issue.

[Zsailer]

BTW, thank you @bollwyvl!

[bollwyvl]

Thanks so much for the quick review! ☺️