Add test case for dropping unsendable class elsewhere.

tests/test_class_basics.rs

@@ -526,3 +526,42 @@ fn access_frozen_class_without_gil() {
 
     assert_eq!(py_counter.get().value.load(Ordering::Relaxed), 1);
 }
+
+#[test]
+fn drop_unsendable_elsewhere() {
+    use std::cell::RefCell;
+    use std::rc::Rc;
+    use std::thread::{current, spawn, ThreadId};
+
+    #[pyclass(unsendable)]
+    struct Unsendable {
+        thread_id: Rc<RefCell<ThreadId>>,
+    }
+
+    impl Drop for Unsendable {
+        fn drop(&mut self) {
+            let created = *self.thread_id.borrow();
+            let dropped = current().id();
+            assert_eq!(
+                created, dropped,
+                "I was created on {created:?}, but dropped on {dropped:?}."
+            );
+        }
+    }
+
+    let unsendable = Python::with_gil(|py| {
+        Py::new(
+            py,
+            Unsendable {
+                thread_id: Rc::new(RefCell::new(current().id())),
+            },
+        )
+        .unwrap()
+    });
+
+    spawn(move || {
+        Python::with_gil(move |_py| {
+            drop(unsendable);
+        });
+    });
+}