How do you convert a python time.struct_time object into a ISO string?

Using time.strftime() is perhaps easiest:

iso = time.strftime('%Y-%m-%dT%H:%M:%SZ', timetup)

Demo:

>>> import time
>>> timetup = time.gmtime()
>>> time.strftime('%Y-%m-%dT%H:%M:%SZ', timetup)
'2013-10-11T13:31:03Z'

You can also use a datetime.datetime() object, which has a datetime.isoformat() method:

>>> from datetime import datetime
>>> datetime(*timetup[:6]).isoformat()
'2013-10-11T13:31:03'

This misses the timezone Z marker; you could just add that.