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/output.py

@@ -24,10 +24,14 @@ Simple description of return values.
 from inspect import getdoc
 from types import NoneType
 
+import six
+
 from ipalib.plugable import ReadOnly, lock
 from ipalib.capabilities import client_has_capability
 from ipalib.text import _
 
+if six.PY3:
+    unicode = str
 
 class Output(ReadOnly):
     """