Since Python 3 it must be "utf-8", which is the official name.
This is backwards compatible with Python 2.

Bug: 962277
Change-Id: I21115a322944532f1870862c8c048f8455c1109f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1608562
Auto-Submit: Raul Tambre <raul@tambre.ee>
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>