cros: switch CrOS programs to python3
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We start warning if people have outdated systems.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 <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Reviewed-by: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>

CoherentLabs/depot_tools,CoherentLabs/depot_tools