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>

ipalib/text.py

@@ -120,6 +120,9 @@ import six
 
 from ipalib.request import context
 
+if six.PY3:
+    unicode = str
+
 
 def create_translation(key):
     assert key not in context.__dict__