This will make `vpython3` explicitly opt-in (so it will now behave the
same way that it does on bots).

R=qyearsley@chromium.org, smut@google.com, tandrii@chromium.org, vadimsh@chromium.org

Bug: 1051595
Change-Id: I80cbc640ec76756381d8b1cc2755363180534b96
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2072361
Auto-Submit: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Quinten Yearsley <qyearsley@chromium.org>
Reviewed-by: Quinten Yearsley <qyearsley@chromium.org>