UBSan with -fsanitize=implicit-integer-truncation found a suspicious one:
/scratch2/izbyshev/cpython/Modules/cjkcodecs/_codecs_jp.c:43:17: runtime error: implicit conversion from type 'unsigned int' of value 4294966013 (32-bit, unsigned) to type 'unsigned char' changed the value to 253 (8-bit, unsigned)

Indeed, the wrong constant was used (the correct one is used in corresponding decoder code at https://github.com/python/cpython/blob/fd512d76456b65c529a5bc58d8cfe73e4a10de7a/Modules/cjkcodecs/_codecs_jp.c#L105). In this case the truncation was harmless because only the lowest byte of the wrong result was used, and it was correct. But it probably makes sense to fix it if only to reduce noise from UBSan.

All Python versions are affected, but I've marked 3.8 only since I'm not sure what the policy for backporting such changes is.