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_ssh.py

@@ -21,10 +21,16 @@ Test the `ipapython/ssh.py` module.
 """
 
 import base64
+
+import six
 import nose
 
 from ipapython import ssh
 
+if six.PY3:
+    unicode = str
+
+
 def make_public_key_checker(pk, out):
     def check_public_key():
         try: