Py3: Replace six.string_types with str

In Python 3, six.string_types is just an alias for str.

See: https://pagure.io/freeipa/issue/7715
Signed-off-by: Christian Heimes <cheimes@redhat.com>
Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>

ipalib/messages.py

@@ -71,7 +71,7 @@ def process_message_arguments(obj, format=None, message=None, **kw):
             obj.format = format
         obj.forwarded = False
         obj.msg = obj.format % kw
-        if isinstance(obj.format, six.string_types):
+        if isinstance(obj.format, str):
             obj.strerror = ugettext(obj.format) % kw
         else:
             obj.strerror = obj.format % kw