Py3: Replace six.moves imports

Replace six.moves and six.StringIO/BytesIO imports with cannonical
Python 3 packages.

Note: six.moves.input behaves differently than builtin input function.
Therefore I left six.moves.input for now.

See: https://pagure.io/freeipa/issue/7715
Signed-off-by: Christian Heimes <cheimes@redhat.com>
Reviewed-By: Fraser Tweedale <ftweedal@redhat.com>

ipatests/test_cmdline/test_help.py

@@ -19,11 +19,11 @@
 
 import sys
 import os
+from io import StringIO
 import shutil
 import errno
 
 import six
-from six import StringIO
 
 from ipalib import api, errors
 from ipaserver.plugins.user import user_add