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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 is_unicode_string(string):
 """ Return ``True`` if the given string is a Unicode string, that is, of type ``unicode`` in Python 2 or ``str`` in Python 3. Return ``None`` if ``string`` is ``None``. :param str string: the string to be checked :rtype: bool """

    if string is None:
        return None
    if PY2:
        return isinstance(string, unicode)
    return isinstance(string, str)