remove six pkg references

[shasibhusanJena]

Summary

• Remove all usage of "six" in the UST codebase.

• changes are
  • six.iteritems(), we can use items() on the collection
  • six.iterkeys() can be replaced by calling keys() on the collection
  • six.itervalues() can be replaced by calling values() on the collection

Testing Steps

• Run unit test using "pytest".
• Check all application sync process by running application on test mode.

Fixes #760