gh-132617: Fix dict.update() mutation check

[colesbury]

Use ma_used instead of ma_keys->dk_nentries for modification check so that we only check if the dictionary is modified, not if new keys are added to a different dictionary that shared the same keys object.

• Issue: dict.update() mutation check too broad #132617

[methane]

ma_used is too weak. It cannot detect del d["a"]; d["b"] = None.

For better check:

• For all table, check other->ma_keys is not changed.
• For combined table, check other->ma_keys->dk_nentries is not changed.
• For split table, check other->ma_values->size is not changed.

[colesbury]

It's the same condition we use in iterators and elsewhere. I don't think we should strengthen any of the checks. These are heuristic checks, they're not going to be perfect, but if we decide to do so, I think we should do that in a subsequent PR and not backport that change.

cpython/Objects/dictobject.c

Lines 5169 to 5174 in 469a564

	if (di->di_used != d->ma_used) {
	PyErr_SetString(PyExc_RuntimeError,
	"dictionary changed size during iteration");
	di->di_used = -1; /* Make this state sticky */
	return NULL;
	}