add BaseExceptionGroup.__init__, following CPython

[cfbolz]

This was reported to PyPy as an incompatibility in this issue: pypy/pypy#5218

CPython's BaseExceptionGroup.init looks like this:

static int
BaseExceptionGroup_init(PyObject *self, PyObject *args, PyObject *kwds)
{
    if (!_PyArg_NoKeywords(Py_TYPE(self)->tp_name, kwds)) {
        return -1;
    }
    if (BaseException_init(self, args, kwds) == -1) {
        return -1;
    }
    return 0;
}

[agronholm]

I would be much more comfortable with this if we first asked on the CPython tracker if emulating this bug is okay or if they're going to fix it (or willing to accept a patch) over there.