How to use the pypandoc.py3compat.cast_bytes function in pypandoc

p = subprocess.Popen(
        args,
        stdin=subprocess.PIPE if string_input else None,
        stdout=subprocess.PIPE,
        stderr=subprocess.PIPE,
        env=new_env)

    # something else than 'None' indicates that the process already terminated
    if not (p.returncode is None):
        raise RuntimeError(
            'Pandoc died with exitcode "%s" before receiving input: %s' % (p.returncode,
                                                                           p.stderr.read())
        )

    try:
        source = cast_bytes(source, encoding='utf-8')
    except (UnicodeDecodeError, UnicodeEncodeError):
        # assume that it is already a utf-8 encoded string
        pass
    try:
        stdout, stderr = p.communicate(source if string_input else None)
    except OSError:
        # this is happening only on Py2.6 when pandoc dies before reading all
        # the input. We treat that the same as when we exit with an error...
        raise RuntimeError('Pandoc died with exitcode "%s" during conversion.' % (p.returncode))

    try:
        stdout = stdout.decode('utf-8')
    except UnicodeDecodeError:
        # this shouldn't happen: pandoc more or less garantees that the output is utf-8!
        raise RuntimeError('Pandoc output was not utf-8.')