gencodec.py is only rarely used, namely when adding new codecs based
on Unicode mapping files.

It is not run regularly on the files from ftp.unicode.org and only
updated on demand.

AFAIK, it was last used on Python2 and never on Python3, hence the
errors you find with it.

BTW: You appear to have a comma appended to the constant, that doesn't
belong there:

+# Placeholder for a missing codepoint
+MISSING_CODE = -1,
+

Perhaps that's causing the second error you are seeing.