As we update scripts to run Python 3 by default, we want to provide
developers a way to execute scripts on Python 2 if they encounter
any problems.

This change only affects Linux/Mac and not Windows developers.

Bug: 984182
Change-Id: I7c7bd2a45ef50e340f6f2b85e9bc8b63f22d8678
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2079701Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>