Update all .py source files by
$ pyupgrade --py3-only $(git ls-files | grep ".py$")
to modernize the code according to Python 3 syntaxes.
Also add the pyupgrade hook to pre-commit to avoid merging additional
Python 2 syntaxes.
Change-Id: I8df72bc1f372b2ec5663a4662325a5ba353f65f0
f58c67d40f