Augment functools.cached_property tests for set/delete

cached_property supports set and delete but this is not currently tested.

Minimal test extending TestCachedProperty.test_cached:

class TestCachedProperty(unittest.TestCase):
    def test_cached(self):
        item = CachedCostItem()
        self.assertEqual(item.cost, 2)
        self.assertEqual(item.cost, 2) # not 3

        item.cost = 42
        self.assertEqual(item.cost, 42)

        del item.cost
        self.assertEqual(item.cost, 3)
        self.assertEqual(item.cost, 3) # not 4

Linked PRs

• gh-142645
• gh-143393

Start in Lib/test/test_functools.py at TestCachedProperty.test_cached, the location named in the issue. Run the focused functools tests first, then extend this test to cover setting and deleting the cached property as shown. Done means the new assertions pass and the existing cached-property behavior remains covered.