turn some checks into assertions, since they are implied by the caller

Reviewed by Georg.

Objects/typeobject.c

@@ -2945,10 +2945,8 @@ same_slots_added(PyTypeObject *a, PyTypeObject *b)
     Py_ssize_t size;
     PyObject *slots_a, *slots_b;
 
-    if (base != b->tp_base)
-        return 0;
-    if (equiv_structs(a, base) && equiv_structs(b, base))
-        return 1;
+    assert(base == b->tp_base);
+    assert(equiv_structs(a, base) && equiv_structs(b, base));
     size = base->tp_basicsize;
     if (a->tp_dictoffset == size && b->tp_dictoffset == size)
         size += sizeof(PyObject *);