PM-1371 Use six.string_types instead of separate str, unicode in isinstance checks.
PM-1371 Use six.string_types instead of separate str, unicode in isinstance checks.

pegasus-isi/pegasus,pegasus-isi/pegasus,pegasus-isi/pegasus,pegasus-isi/pegasus,pegasus-isi/pegasus,pegasus-isi/pegasus,pegasus-isi/pegasus,pegasus-isi/pegasus,pegasus-isi/pegasus,pegasus-isi/pegasus