[cleanup] Upgrade syntax

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

setup.py

@@ -1,5 +1,4 @@
 #!/usr/bin/env python3
-# coding: utf-8
 import os.path
 import warnings
 import sys