Run pyupgrade to simplify code and use Python 3.6 syntax

[jdufresne]

pyugrade is a command line tool to automatically update Python syntax to
modern usage and patterns. For additional details, see:

https://github.com/asottile/pyupgrade

Changes made by the tool:

• Use short Python3 super() syntax.
• Use f-strings when they are simple and more readable.
• Drop Python 2 u prefix from strings.
• Drop "r" argument from open(). It is the default and so specifying it is
  unnecessary.