iso8601.UTC is correct datetime UTC field object.
iso8601 >= 0.1.12 includes only iso8601.UTC for python3
while both UTC and Utc() for python2. Less then 0.1.12
included both UTC and Utc() for both python2/3.
Change-Id: I0f8796fba6725eea013b3f8d9ad33c10a402c524
Closes-Bug: #1715486