This is a reland of 35b70cac.
With the recent vpython3 PATH fixes, hopefully this should pass now.

Original change's description:
> cros: switch CrOS programs to python3
>
> We start warning if people have outdated systems.
>
> Bug: 997354
> Test: `cros --help` uses python3 now
> Change-Id: I1491f1c0eea4d544cc508b461153c54c8072699d
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2052243
> Commit-Queue: Mike Frysinger <vapier@chromium.org>
> Reviewed-by: Robbie Iannucci <iannucci@chromium.org>

Bug: 997354
Change-Id: Ic7e040af6a34d112cd0a1c1421cdd25bc557ee19
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2052625
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>