Jonathan Goble writes:
I assume the "cpython" part of these paths here is your local clone of the CPython GitHub repo? (Otherwise these local filepaths from your computer don't make sense.)
Thanks for catching that!
Sorry, I was concentrating on stifling irrelevant Unicode politics. :-) You need a local clone of the GitHub repo, and the various possibly relevant files are in
Doc/library/codecs.rst (these two are available online) Doc/c-api/codec.rst
Python/codecs.c Include/codecs.h Objects/stringlib/codecs.h Objects/unicodectype.c Modules/_codecsmodule.c Modules/cjkcodecs Modules/clinic/_codecsmodule.c.h