remove six reference

Author: mvantellingen

src/zeep/xsd/types/builtins.py

@@ -372,7 +372,7 @@ class Base64Binary(BuiltinType):
 
     @check_no_collection
     def xmlvalue(self, value):
-        if isinstance(value, six.string_types):
+        if isinstance(value, str):
             return value
         return base64.b64encode(value)