Rust thread safety in a Python extension

safety_is_key.rs

#[macro_use]
extern crate cpython;
use std::rc::Rc;

pub struct Simulation {
    pub star: Rc<Star>,
    pub spots: Vec<Spot>,
}

pub struct Spot {
    pub star: Rc<Star>,
}

pub struct Star {}

#[doc(hidden)]
pub use py_interface::PyInit_lather;

#[allow(missing_docs)]
mod py_interface {
    use std::cell::RefCell;

    py_module_initializer!(lather, initlather, PyInit_lather, |py, m| {
        m.add_class::<PySimulation>(py)?;
        Ok(())
    });

    py_class!(class PySimulation |py| {
            data sim: RefCell<::Simulation>;
        });
}

// Attempting to build this produces the following error:
/*
error[E0277]: the trait bound `std::rc::Rc<Star>: std::marker::Send` is not satisfied in `Simulation`
  --> src/main.rs:29:5
   |
29 | /     py_class!(class PySimulation |py| {
30 | |             data sim: RefCell<::Simulation>;
31 | |         });
   | |___________^ `std::rc::Rc<Star>` cannot be sent between threads safely
   |
   = help: within `Simulation`, the trait `std::marker::Send` is not implemented for `std::rc::Rc<Star>`
   = note: required because it appears within the type `Simulation`
   = note: required because of the requirements on the impl of `std::marker::Send` for `std::cell::RefCell<Simulation>`
   = note: required by `cpython::py_class::data_init`
   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
*/