Try this:
>>> def gt(dt_str):
... dt, _, us = dt_str.partition(".")
... dt = datetime.datetime.strptime(dt, "%Y-%m-%dT%H:%M:%S")
... us = int(us.rstrip("Z"), 10)
... return dt + datetime.timedelta(microseconds=us)
Usage:
>>> gt("2008-08-12T12:20:30.656234Z")
datetime.datetime(2008, 8, 12, 12, 20, 30, 656234)