use six privided functions/classes to support py2/py3 in one source. refs #1350.

tests/etree13/ElementTree.py

@@ -81,6 +81,9 @@
 from __future__ import generators
 from __future__ import absolute_import
 
+import six
+
+
 __all__ = [
     # public symbols
     "Comment",
@@ -828,7 +831,7 @@ def _namespaces(elem, encoding, default_namespace=None):
         tag = elem.tag
         if isinstance(tag, QName) and tag.text not in qnames:
             add_qname(tag.text)
-        elif isinstance(tag, basestring):
+        elif isinstance(tag, six.string_types):
             if tag not in qnames:
                 add_qname(tag)
         elif tag is not None and tag is not Comment and tag is not PI: