The newest pylint release complained about several issues that the
older release did not. This resolves those issues:
- removes unneeded "u" prefix from strings
- adds "encoding" parameter for all open() calls
- because "encoding" is a python3-only parameter, use python3 in all
the scripts that we control
Unfortunately, python2 is required for any scripts that import modules
from the ancient Chromium build system we're using (referenced by
DEPS), as well as kokoro scripts.
Change-Id: I2e9f97af508efe58b5a71de21740e59b1528affd
ac125564b9