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/plugins/cert.py

@@ -40,10 +40,15 @@ from ipalib.text import _
 from ipalib.request import context
 from ipalib import output
 from ipalib.plugins.service import validate_principal
+
+import six
 import nss.nss as nss
 from nss.error import NSPRError
 from pyasn1.error import PyAsn1Error
 
+if six.PY3:
+    unicode = str
+
 __doc__ = _("""
 IPA certificate operations