Using https://github.com/asottile/pyupgrade
1. `__future__` imports and `coding: utf-8` were removed
2. Files were rewritten with `pyupgrade --py36-plus --keep-percent-format`
3. f-strings were cherry-picked from `pyupgrade --py36-plus`
Extractors are left untouched (except removing header) to avoid unnecessary merge conflicts
From now on, the line
from __future__ import unicode_literals
should be contained in every single Python file lest we run into any more 2.x/3.x issues.
Going forward, we're likely to develop on 3.x only and would likely miss subtle bugs otherwise.
