Drop python 2 support

[jasongrout]

Python 2 has end-of-life on January 1, 2020. Accordingly, a few of us feel that we should make an 8.0 ipywidgets release that moves to Python 3 only before then.

[vidartf]

I agree. Let's use this issue as a tracker for things to refactor as a part of that switch. E.g. "change this API to use async generator"; "change this internal code to cleanup bytestring/unicode use", etc.

[hugovk]

Please see PR #2558 to take care of some of this.