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OK, a few notes on the patch:

In the test "c = codecs.lookup('utf-8')[3](s)" should be
written as "c = codecs.getwriter('utf-8')(s)". Someday in
the future CodecInfo may no longer be a tuple.

Instead of htmlentitydefs.entitydefs, the test could use
htmlentitydefs.name2codepoint.

I'd like to see a few tests for error callbacks that return
the wrong objects and for callbacks that return an offset
that is not exc.end.

The ERROR_ISCUSTOM() macro looks wrong to me: smaller than 1
*and* greater that 3?

In mbiencoder_encode()

r = encoder_encode_stateful(STATEFUL_ECTX(self), data, final);
if (r == NULL)
    return NULL;
return r;

can be simplified to:

return encoder_encode_stateful(STATEFUL_ECTX(self), data,
final);

Apart from that the patch looks good to me, so go ahead and
check it in.