au_test_harness: update to python3

BUG=chromium:1170006
TEST=CQ passes

Change-Id: I3a269ba7315049a148daaf973922003877cd6a67
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crostestutils/+/2689376
Reviewed-by: Sean Abraham <seanabraham@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
8 files changed