Argument Clinic: inline the parsing code of optional groups #155373

Optional groups are the last parsing method in Argument Clinic which still calls the runtime parser: it generates one PyArg_ParseTuple() call per number of arguments, in a big switch.

The code of the converters can be inlined, as in all other methods, with every argument parsed once. For _curses.window.addch([y, x,] ch, [attr]) the four PyArg_ParseTuple() calls become:

    if (nargs >= 3) {
        y = PyLong_AsInt(args[0]);
        ...
        offset += 2;
        group_left_1 = 1;
    }
    ch = args[offset];
    if (nargs == 2 || nargs == 4) {
        if (!attr_converter(args[nargs - 1], &attr)) {
            goto exit;
        }
        group_right_1 = 1;
    }

This also allows using the fastcall convention, except for constructors and other functions which need a tuple.

Errors for a wrong type of an argument become the same as in other functions, because they are now reported by the converter.

Linked PRs

• gh-155373: Inline the parsing code of optional groups in Argument Clinic #155374