cefbuilds: Add Python 3 support (see issue #2856)

tools/cefbuilds/cef_json_builder_test.py

@@ -2,6 +2,7 @@
 # reserved. Use of this source code is governed by a BSD-style license that
 # can be found in the LICENSE file.
 
+from __future__ import absolute_import
 from cef_json_builder import cef_json_builder
 import datetime
 import unittest