How to make extract work for a type wrapping PyAny

[flying-sheep]

I want to create the following type wrapping scipy.sparse.sp{matrix,array}:

struct CSMatrix<I, D>(PyAny, PhantomData<I>, PhantomData<D>);

I want to avoid having an internal reference inside since it should only be used through Bound<'py, CSMatrix<_, _>>.

How can I get this to work:

fn test(ob: Bound<'py, PyAny>) -> PyResult<...> {
    let mat: CSMatrix<usize, f64> = ob.extract()?;
    ...
}