add coding:utf-8 to all python files

The linter wants this to make sure we use UTF-8 encoding everywhere.

BUG=None
TEST=precq passes

Change-Id: Id63f57f0b3ef25e04b02a94adae2f17cb1d34214
Reviewed-on: https://chromium-review.googlesource.com/1497252
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>