Update all .py source files by
$ pyupgrade --py3-only $(git ls-files | grep ".py$")
to modernize the code according to Python 3 syntaxes.
pep8 errors are fixed by manual adjustments.
Also add the pyupgrade hook to pre-commit to avoid merging additional
Python 2 syntaxes.
Change-Id: I14cd7c88346bdfb5d4b2afe2b4a71f84833f3688
de05ceca5f