Add missing pub classifier

Anyone copy-pasting the code and trying to use the new `@shared` interface for the first time will get an error about leaking a private type.

src/sharedref.rs

@@ -54,7 +54,7 @@ use crate::python::{PyClone, Python};
 ///     }
 /// });
 ///
-/// py_class!(class ListIterator |py| {
+/// py_class!(pub class ListIterator |py| {
 ///     data rust_iter: RefCell<UnsafePyLeaked<Iter<'static, i32>>>;
 ///
 ///     def __next__(&self) -> PyResult<Option<PyInt>> {