tauto: Update presubmit to use python3

BUG=None
TEST=presubmit on repo upload

Change-Id: I466bc896089b714a38a318cc2013013e190e4d3c
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/tauto/+/4119770
Reviewed-by: Derek Beckett <dbeckett@chromium.org>
Auto-Submit: Jesse McGuire <jessemcguire@google.com>
Tested-by: Jesse McGuire <jessemcguire@google.com>
Reviewed-by: Jesse McGuire <jessemcguire@google.com>
Commit-Queue: Derek Beckett <dbeckett@chromium.org>
Commit-Queue: Jesse McGuire <jessemcguire@google.com>