Drop support for Python 2.7

[rolweber]

Python 2.7 reaches its end of life at new year's eve.
Should we still bother with it in jupyter_server until then?

I'm in favor of dropping it rather sooner than later. Should be just some metadata and CI configuration that needs to be updated.

[kevin-bates]

This issue was probably written up before the Notebook 6 requirements updates, but since Notebook doesn't support anything less that Python 3.5, I think jupyter_server should follow suit.

I'd be happy to work on this if others are okay with that.