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>

ipapython/ipaldap.py

@@ -372,7 +372,7 @@ class LDAPEntry(MutableMapping):
             self._not_list.discard(name)
 
     def _attr_name(self, name):
-        if not isinstance(name, six.string_types):
+        if not isinstance(name, str):
             raise TypeError(
                 "attribute name must be unicode or str, got %s object %r" % (
                     name.__class__.__name__, name))