AST type doesn't have any information about optional fields.  And I don't know it's worth enough to add it.
When AST is instantiated by keyword arguments, there are no check about absence of some fields.
I suppose we can just loosen positional arguments too.

current:

    if (PyTuple_GET_SIZE(args) > 0) {
        if (numfields != PyTuple_GET_SIZE(args)) {
            PyErr_Format(PyExc_TypeError, "%.400s constructor takes %s"
                         "%zd positional argument%s",
                         Py_TYPE(self)->tp_name,
                         numfields == 0 ? "" : "either 0 or ",
                         numfields, numfields == 1 ? "" : "s");

will be:

    if (PyTuple_GET_SIZE(args) > 0) {
        if (numfields > PyTuple_GET_SIZE(args)) {
            PyErr_Format(PyExc_TypeError, "%.400s constructor takes at most "
                         "%zd positional argument%s",
                         Py_TYPE(self)->tp_name,
                         numfields, numfields == 1 ? "" : "s");