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I'm not sure this is correct: unless the codecs implement
their own .readline() implementation, the one in codecs.py
is used and that simply delegates the readline request to
the underlying stream object.

Now. since the stream object in the source code reader is
a plain Python file object, currently opened in "rb" mode,
changing the mode to "rbU" should be enough to get
universal readline support for all such codecs.

The relevant code is in Parser/tokenizer.c:fp_setreadl():

static int
fp_setreadl(struct tok_state *tok, const char* enc)
{
	PyObject *reader, *stream, *readline;

	/* XXX: constify filename argument. */
	stream = PyFile_FromFile(tok->fp, (char*)tok->filename,
"rb", NULL);
	if (stream == NULL)
		return 0;

	reader = PyCodec_StreamReader(enc, stream, NULL);
	Py_DECREF(stream);
	if (reader == NULL)
		return 0;

	readline = PyObject_GetAttrString(reader, "readline");
	Py_DECREF(reader);
	if (readline == NULL)
		return 0;

	tok->decoding_readline = readline;
	return 1;
}