Run gclient, roll-dep, fetch and custom git commands (i.e. git-cl, git-rebase-update, git-new-branch, etc.)
using vpython3 by default.

Change-Id: I4eecddafa6ca4c5f82ec097615c79d2a741613e7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2113550Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>