d will convert a
    cStringIO into an L{io.StringIO}.

    @param val: The content of the file.
    @type val: L{bytes}

    @param sek: The seek position of the file.
    @type sek: L{int}

    @return: a file-like object which you can write bytes to.
    @rtype: L{cStringIO.OutputType} on Python 2, L{io.StringIO} on Python 3.
    )