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_ipapython/test_dn.py

@@ -2,8 +2,13 @@
 
 import unittest
 
+import six
+
 from ipapython.dn import *
 
+if six.PY3:
+    unicode = str
+
 def expected_class(klass, component):
     if klass is AVA:
         if component == 'self':