error[E0277]: `Rc<i32>` cannot be sent between threads safely
   --> $DIR/pyclass_send.rs:4:1
    |
4   | #[pyclass]
    | ^^^^^^^^^^ `Rc<i32>` cannot be sent between threads safely
    |
   ::: $WORKSPACE/src/pyclass.rs
    |
    | pub struct ThreadCheckerStub<T: Send>(PhantomData<T>);
    |                                 ---- required by this bound in `ThreadCheckerStub`
    |
    = help: within `NotThreadSafe`, the trait `Send` is not implemented for `Rc<i32>`
    = note: required because it appears within the type `NotThreadSafe`
    = note: this error originates in an attribute macro (in Nightly builds, run with -Z macro-backtrace for more info)