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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 get_iso_time_str(timestamp: Union[int, float, str, datetime]=None) -> str: """Get the ISO time string from a timestamp or date obj. Returns current time str if no timestamp is passed"""

    if isinstance(timestamp, (int, float)):
        maya_dt = maya.MayaDT(timestamp)
    elif isinstance(timestamp, str):
        maya_dt = maya.when(timestamp)
    elif timestamp is None:
        maya_dt = maya.now()
    else:
        raise ValueError(f'`{type(timestamp)}` is not supported')
    return maya_dt.iso8601()