[Autotest] /server/cros py3 migration

BUG=chromium:990593
TEST=dummy_Pass, python3 py_compile, python py_compile, unittests

Change-Id: I510a54e18087bce69e3c0acc3e7489d160ee7dc8
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/autotest/+/2319151
Tested-by: Derek Beckett <dbeckett@chromium.org>
Commit-Queue: Derek Beckett <dbeckett@chromium.org>
Reviewed-by: Ruben Zakarian <rzakarian@chromium.org>

server/cros/camerabox_utils.py

diff --git a/server/cros/camerabox_utils.py b/server/cros/camerabox_utils.py
index 0f222f8..b3c7f8c 100644
--- a/server/cros/camerabox_utils.py
+++ b/server/cros/camerabox_utils.py
@@ -1,3 +1,4 @@
+# Lint as: python2, python3
 # Copyright 2018 The Chromium OS Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
@@ -7,7 +8,7 @@
 import logging
 from lxml import etree
 import os
-import StringIO
+import six
 import time
 
 from autotest_lib.client.common_lib import error, utils
@@ -111,7 +112,7 @@
             renumbered from 0.
         """
         tree = etree.parse(
-                StringIO.StringIO(content),
+                six.StringIO(content),
                 parser=etree.XMLParser(compact=False))
         root = tree.getroot()
         profiles = root.findall('CamcorderProfiles')