3rd round of the API:

    static PyObject *assertion_error = NULL;
    ...
    if (_PY_ONCEVAR_INIT(assertion_error,
                         PyUnicode_InternFromString("AssertionError"))) {
        return 0;
    }
    ...

(Not the best example, _Py_IDENTIFIER() would be more appropriate here ;-))

--

I just added a second commit to remove the next field from _Py_Identifier and reuse _PyOnceVar_Set() in _PyUnicode_FromId().