Please fold these cases into one:

     if (strcmp(buf, "utf-8") == 0 ||
         strncmp(buf, "utf-8-", 6) == 0)
         return "utf-8";
     else if (strcmp(buf, "utf8") == 0 ||
         strncmp(buf, "utf8-", 6) == 0)
         return "utf-8";

->

     if (strcmp(buf, "utf-8") == 0 ||
         strncmp(buf, "utf-8-", 6) == 0 ||
         strcmp(buf, "utf8") == 0 ||
         strncmp(buf, "utf8-", 6) == 0)
         return "utf-8";

Also: I wonder why the regular utf_8.py codec doesn't complain about this case, since the above are only shortcuts for frequently used source code encodings.