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: I2adc97da3ef11ee18e30ed92e9ca5992d384fdb1
822f3bd94b