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_cmdline/test_help.py

@@ -22,10 +22,14 @@ import contextlib
 import StringIO
 
 from nose.tools import assert_raises  # pylint: disable=E0611
+import six
 
 from ipalib import api, errors
 from ipalib.plugins.user import user_add
 
+if six.PY3:
+    unicode = str
+
 
 class CLITestContext(object):
     """Context manager that replaces stdout & stderr, and catches SystemExit