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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 from_iso_datetime(datetimestring, use_dateutil=True):
 """Parse an ISO8601-formatted datetime string and return a datetime object. Use dateutil's parser if possible and return a timezone-aware datetime. """

    if not _iso8601_datetime_re.match(datetimestring):
        raise ValueError('Not a valid ISO8601-formatted datetime string')
    # Use dateutil's parser if possible
    if dateutil_available and use_dateutil:
        return parser.isoparse(datetimestring)
    else:
        # Strip off timezone info.
        return datetime.datetime.strptime(datetimestring[:19], '%Y-%m-%dT%H:%M:%S')