bpo-33029: Fix signatures of getter and setter functions. (GH-10746)

Fix also return type for few other functions (clear, releasebuffer).
diff --git a/Modules/cjkcodecs/multibytecodec.c b/Modules/cjkcodecs/multibytecodec.c
index 8a0ac87..938a774 100644
--- a/Modules/cjkcodecs/multibytecodec.c
+++ b/Modules/cjkcodecs/multibytecodec.c
@@ -108,7 +108,7 @@
 }
 
 static PyObject *
-codecctx_errors_get(MultibyteStatefulCodecContext *self)
+codecctx_errors_get(MultibyteStatefulCodecContext *self, void *Py_UNUSED(ignored))
 {
     const char *errors;