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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 isotime(at=None, subsecond=False):
 """Stringify time in ISO 8601 format."""

    if not at:
        at = utcnow()
    st = at.strftime(_ISO8601_TIME_FORMAT
                     if not subsecond
                     else _ISO8601_TIME_FORMAT_SUBSECOND)
    tz = at.tzinfo.tzname(None) if at.tzinfo else 'UTC'
    st += ('Z' if tz == 'UTC' else tz)
    return st