bpo-1635741: Convert _multibytecodec to multi-phase init (GH-24095)

Convert the _multibytecodec extension module (CJK codecs) to multi-phase
initialization (PEP 489).
diff --git a/Modules/cjkcodecs/multibytecodec.h b/Modules/cjkcodecs/multibytecodec.h
index 5946821..69404ba 100644
--- a/Modules/cjkcodecs/multibytecodec.h
+++ b/Modules/cjkcodecs/multibytecodec.h
@@ -65,7 +65,7 @@ typedef struct {
     MultibyteCodec *codec;
 } MultibyteCodecObject;
 
-#define MultibyteCodec_Check(op) Py_IS_TYPE((op), &MultibyteCodec_Type)
+#define MultibyteCodec_Check(state, op) Py_IS_TYPE((op), state->multibytecodec_type)
 
 #define _MultibyteStatefulCodec_HEAD            \
     PyObject_HEAD                               \