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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def convert_iso_stamp(t, tz=None):
 """Convert a string in ISO8601 form into a Datetime object. This is mainly used for converting timestamps sent from the TempoDB API, which are assumed to be correct. :param string t: the timestamp to convert :rtype: Datetime object"""


    if t is None:
        return None

    dt = dateutil.parser.parse(t)
    if tz is not None:
        timezone = pytz.timezone(tz)
        if dt.tzinfo is None:
            dt = timezone.localize(dt)
    return dt