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/cli.py

@@ -36,6 +36,9 @@ import traceback
 import six
 from six.moves import input
 
+if six.PY3:
+    unicode = str
+
 try:
     #pylint: disable=F0401
     import default_encoding_utf8