This matches the recent change Chromium's Python style guide.
This also changes the .style.yapf in depot_tools to be explicit
about the style used in this repo.

Bug: 846432
Change-Id: I4947a90aec1739a9a86ffc9bfc5eacc1182dc186
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1987006Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>