Alias "unicode" to "str" under Python 3

The six way of doing this is to replace all occurences of "unicode"
with "six.text_type". However, "unicode" is non-ambiguous and
(arguably) easier to read. Also, using it makes the patches smaller,
which should help with backporting.

Reviewed-By: Petr Viktorin <pviktori@redhat.com>

ipatests/test_util.py

@@ -23,9 +23,14 @@ Test the `tests.util` module.
 
 import re
 
+import six
+
 from ipatests import util
 from ipatests.util import raises, TYPE, VALUE, LEN, KEYS
 
+if six.PY3:
+    unicode = str
+
 
 class Prop(object):
     def __init__(self, *ops):