The iso8601 lib introduced a change such that if running on python
3.2 or later it internally uses the python timezone information
instead of its own implementation. This does not change direct
date handling, but when converting this value there is a slight
difference where now python 2.x will show UTC times as "UTC",
but on python 3 they will end up with "UTC+00:00".
The to_primitive call for DateTime fields was doing an exact match
on "UTC" to determine whether to include "Z" in the resulting string.
This updates that handling to recognize either of the new values
Change-Id: Icb29d71472932f3ddfe485298d1a5fdd08be6f12
Closes-bug: #1744160
daa3c88d8b