message: Convert unicode to six.string_types for python 3
  In Python 3, there is no unicode type. This fix converts
  unicode to use six.string_types instead, while maintaining
  python 2/3 compatibility.
diff: @@ -879,7 +879,7 @@ class GraphDump(BaseMonitor):
  this_output = self.data[step] if step in self.data else {}
  other_output = other_dump.data[step] if step in other_dump.data else {}
  for key in this_output:
  - if not isinstance(key, str) and not isinstance(key, unicode):
  + if not isinstance(key, six.string_types):
  continue
  if key not in other_output:
  raise ValueError("%s missing at step %s.", (key, step))
  