GH-95707: Fix uses of Py_TPFLAGS_MANAGED_DICT (GH-95854)

* Make sure that tp_dictoffset is correct with Py_TPFLAGS_MANAGED_DICT is set.

* Avoid traversing managed dict twice when subclassing class with Py_TPFLAGS_MANAGED_DICT set.

Objects/object.c

@@ -1064,6 +1064,7 @@ _PyObject_ComputedDictPointer(PyObject *obj)
     if (dictoffset == 0)
         return NULL;
     if (dictoffset < 0) {
+        assert(dictoffset != -1);
         Py_ssize_t tsize = Py_SIZE(obj);
         if (tsize < 0) {
             tsize = -tsize;