Preserve __dict__ and __slots__ state in deque.__reduce__ (#7699)

deque.__reduce__ passed None as the unpickle state, so a deque
subclass's __dict__ and __slots__ values were dropped across a pickle
round-trip. CPython's deque___reduce___impl
(Modules/_collectionsmodule.c::deque___reduce___impl) calls
_PyObject_GetState, which returns the dict (or a (dict, slots) tuple)
so subclass attributes survive.

Replace the placeholder with the result of __getstate__() on the
instance. object.__getstate__ already implements the dict / dict+slots
protocol, matching _PyObject_GetState.

Lib/test/test_deque.py

@@ -817,7 +817,6 @@ class TestSubclass(unittest.TestCase):
         d.clear()
         self.assertEqual(len(d), 0)
 
-    @unittest.expectedFailure  # TODO: RUSTPYTHON; AttributeError: 'Deque' object has no attribute 'x'
     def test_copy_pickle(self):
         for cls in Deque, DequeWithSlots:
             for d in cls('abc'), cls('abcde', maxlen=4):

crates/vm/src/stdlib/_collections.rs

@@ -388,7 +388,11 @@ mod _collections {
                 Some(v) => vm.new_pyobj((vm.ctx.empty_tuple.clone(), v)),
                 None => vm.ctx.empty_tuple.clone().into(),
             };
-            Ok(vm.new_pyobj((cls, value, vm.ctx.none(), PyDequeIterator::new(zelf))))
+            // Use __getstate__ to capture both __dict__ and __slots__ values so
+            // subclass attributes survive a pickle round-trip (matches CPython's
+            // deque___reduce___impl, which calls _PyObject_GetState).
+            let state = vm.call_method(zelf.as_object(), "__getstate__", ())?;
+            Ok(vm.new_pyobj((cls, value, state, PyDequeIterator::new(zelf))))
         }
 
         #[pyclassmethod]