This reverts commit 35b70cac.

Reason for revert: causing problems for the chromium cq, e.g.
https://ci.chromium.org/p/chromium/builders/try/chromeos-amd64-generic-rel/471366
https://ci.chromium.org/p/chromium/builders/try/chromeos-arm-generic-rel/197740

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 We start warning if people have outdated systems.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>

TBR=vapier@chromium.org,iannucci@chromium.org,saklein@chromium.org,infra-scoped@luci-project-accounts.iam.gserviceaccount.com

Change-Id: Ie85691d9f847ad7edb4c9932a0788431c886fec2
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 997354
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2052543Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: John Budorick <jbudorick@chromium.org>