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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 to_str(value):
 """Convert the input to a string, unless it is a unicode string in Python 2. Unicode strings are supported as native strings in Python 3, but ``str()`` cannot be invoked on unicode strings in Python 2, so we need to check for that case when converting user-specified values to strings. Args: value: The value to convert to a string. Returns: str or unicode: The string representation of the value or the unicode string itself. """

    if sys.version_info.major < 3 and isinstance(value, six.string_types):
        return value
    return str(value)