py34: fix basestring usage in validator

Change-Id: Ia8ab9435e5eda29d2ff429152a06e5c77ab2a0e4

ec2api/api/validator.py

@@ -16,6 +16,7 @@ import re
 
 import netaddr
 from oslo_log import log as logging
+import six
 
 from ec2api import exception
 from ec2api.i18n import _
@@ -25,7 +26,7 @@ LOG = logging.getLogger(__name__)
 
 
 def validate_str(val, parameter_name, max_length=None):
-    if (isinstance(val, basestring) and
+    if (isinstance(val, six.string_types) and
             (max_length is None or max_length and len(val) <= max_length)):
         return True
     raise exception.ValidationError(