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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 iso_string_to_python_datetime( isostring: str) -> Optional[datetime.datetime]: """ Takes an ISO-8601 string and returns a ``datetime``. """

    if not isostring:
        return None  # if you parse() an empty string, you get today's date
    return dateutil.parser.parse(isostring)