behavior for Python 2. It will decode the
    byte string as ASCII. In Python 3 it simply raises a L{TypeError} when
    passing a byte string. Unicode strings are returned as-is.

    @param s: The string to coerce.
    @type s: L{bytes} or L{unicode}

    @raise UnicodeError: The input L{bytes} is not ASCII decodable.
    @raise TypeError: The input is L{bytes} on Python 3.
    z