'Convert an ISO 8601 formatted string to a datetime object.
We assume that the ISO 8601 formatted string is in UTC and we create
the datetime object so that it is timezone-aware.
:param str time_str: ISO 8601 formatted string
:returns: timezone-aware datetime object
:rtype: datetime or None'

@classmethod
def _strptime(cls, time_str):

    if time_str:
        dt = datetime.strptime(time_str, __timeformat__)
        return dt.replace(tzinfo=UTC())
    return None
