The `Arc<Mutex<T>>` approach here would make me worried about reference cycles, unless the type relationships here look like a tree/DAG. Out of curiosity, if I

The `Arc<Mutex<T>>` approach here would make me worried about reference cycles, unless the type relationships here look like a tree/DAG. Out of curiosity, if I store references to other things as opaque `pyo3::PyObject` objects, would the CPython cycle collector be able to see these references and collect their cycles?