The relevant code in Parser/parsetok.c is:

#ifdef PY_PARSER_REQUIRES_FUTURE_KEYWORD
        if (type == NOTEQUAL) {
            if (!(ps->p_flags & CO_FUTURE_BARRY_AS_BDFL) &&
                            strcmp(str, "!=")) {
                err_ret->error = E_SYNTAX;
                break;
            }
            else if ((ps->p_flags & CO_FUTURE_BARRY_AS_BDFL) &&
                            strcmp(str, "<>")) {
                err_ret->text = "with Barry as BDFL, use '<>' "
                                "instead of '!='";
                err_ret->error = E_SYNTAX;
                break;
            }
        }
#endif


Hmm... I'm too new to remember this joke, but here it presents a practical problem - since Grammar/Grammar is being directly reflected into the documentation (.. literalinclude:: ../../Grammar/Grammar) and thus can confuse.

Is it safe to just remove the whole thing from Grammar/Grammar and correspondingly Parser/parsetok.c ?