Remove "six" dependency from UST

[shasibhusanJena]

Is your feature request related to a problem? Please describe.
Since we no longer target 2.7 for our UST builds, there is no need to continue using this "six" library. So Remove all usage of "six" in the UST codebase

Describe the solution you'd like
Modify use of below existing code to new type

• 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

along with other references of "six" pkg