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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 parse_datetime(__string: str) -> datetime.datetime: """Parse ISO-8601 datetime string. Args: __string: Datetime string to parse Returns: Parsed datetime object """

    if not __string:
        datetime_ = datetime.datetime.now(datetime.timezone.utc)
    else:
        # pylint: disable=no-member
        datetime_ = ciso8601.parse_datetime(__string)
    if datetime_.tzinfo is None:
        datetime_ = datetime_.replace(tzinfo=datetime.timezone.utc)
    return datetime_