bpo-39943: Add the const qualifier to pointers on non-mutable PyUnicode data. (GH-19345)

diff --git a/Modules/cjkcodecs/cjkcodecs.h b/Modules/cjkcodecs/cjkcodecs.h
index 8f6f880..e41755b 100644
--- a/Modules/cjkcodecs/cjkcodecs.h
+++ b/Modules/cjkcodecs/cjkcodecs.h
@@ -72,7 +72,7 @@
 #define ENCODER(encoding)                                               \
     static Py_ssize_t encoding##_encode(                                \
         MultibyteCodec_State *state, const void *config,                \
-        int kind, void *data,                          \
+        int kind, const void *data,                                     \
         Py_ssize_t *inpos, Py_ssize_t inlen,                            \
         unsigned char **outbuf, Py_ssize_t outleft, int flags)
 #define ENCODER_RESET(encoding)                                         \