We have vpython wheels for py3 but not for py2 on mac/arm. With this,
download_from_google_storage.py works on arm mac even without the
vpython bypass. (Also, moving from py2 to py3 seems like a good thing
in general.)

Bug: 1205263,1103236
Change-Id: I6cb08b7c56e5e6fc160fb5abc86d68e04a01f8de
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2881917
Auto-Submit: Nico Weber <thakis@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@google.com>
Reviewed-by: Dirk Pranke <dpranke@google.com>