trivial: Resolve Python 3 issues

Resolve the following exception:

  AttributeError: 'str' object has no attribute 'decode'

This wasn't caught in the gate because that still uses Python 2.7. This
is not the case in other environments (Fedora 27).

Change-Id: I6b462f19432c4cf9948cddcc095d9d5a85feeb4b

test-requirements.txt

@@ -3,3 +3,4 @@
 # process, which may cause wedges in the gate later.
 
 hacking>=0.12.0,!=0.13.0,<0.14  # Apache-2.0
+oslo.utils>=3.33.0 # Apache-2.0

tests/test_titles.py

@@ -15,6 +15,7 @@ import glob
 import re
 
 import docutils.core
+from oslo_utils import encodeutils
 import testtools
 
 
@@ -102,7 +103,7 @@ class TestTitles(testtools.TestCase):
             (len(matches), tpl))
 
     def _check_no_zwsp(self, tpl, raw):
-        matches = raw.decode('utf-8').find(u"\u200B")
+        matches = encodeutils.safe_decode(raw).find(u"\u200B")
         self.assertEqual(
             matches, -1,
             "Found zero width space characters in file %s" %