I tried to implement a custom extension type using PyType_FromSpec and Py_LIMITED_API but couldn't implement tp_dealloc:

static void mytype_dealloc(mytypeobject *self)
{
    // free some fields in mytypeobject
    Py_TYPE(self)->tp_free((PyObject *) self); // XXX
}

The line marked XXX will not compile in Py_LIMITED_API because there is no access to the fields of PyTypeObject.  There doesn't seem to be any function in the API which just calls tp_free.

I suggest the addition of an API function (to be included in Py_LIMITED_API):

void
PyType_GenericDealloc(PyObject *self)
{
    Py_TYPE(self)->tp_free(self);
}