crostestutils: Fix lint errors in files under au_test_harness and ctest

The result is `cros lint au_test_harness/*.py` and `cros lint ctest/*.py` don't
complain. There are over 900 lint errors in files under other directories, but I
don't forsee myself touching any of them in the near future, so I am leaving
them alone...

BUG=None
TEST=`cros lint`, manual ctest run, and trybot

Change-Id: I0ac117a442d652bafe9a11488090181a945e2697
Reviewed-on: https://chromium-review.googlesource.com/325393
Commit-Ready: Daniel Wang <wonderfly@google.com>
Tested-by: Daniel Wang <wonderfly@google.com>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
diff --git a/lib/constants.py b/lib/constants.py
index 9c01596..f8f7055 100644
--- a/lib/constants.py
+++ b/lib/constants.py
@@ -2,6 +2,10 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+"""Module containing contants to be used across crostestutils."""
+
+from __future__ import print_function
+
 import os
 
 _TEST_LIB_PATH = os.path.realpath(__file__)