Chrome is far enough though its Py2->Py3 transition that it makes sense
to change the default here. The default is used whenever a .py file does
not have a shebang at the top of it.

Bug: 1062053
Change-Id: Ie8472f751c1366248d7797a0aaae36c836d9c426
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3282072
Auto-Submit: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Dirk Pranke <dpranke@google.com>