demonstrate switching to Bound API resolves gevent crash

pytests/src/misc.rs

@@ -18,7 +18,11 @@ fn accepts_bool(val: bool) -> bool {
 }
 
 #[pyfunction]
-fn get_item_and_run_callback(dict: &PyDict, callback: &PyAny) -> PyResult<()> {
+fn get_item_and_run_callback(dict: Bound<'_, PyDict>, callback: Bound<'_, PyAny>) -> PyResult<()> {
+    // This function gives the opportunity to run a pure-Python callback so that
+    // gevent can instigate a context switch. This had problematic interactions
+    // with PyO3's removed "GIL Pool".
+    // For context, see https://github.com/PyO3/pyo3/issues/3668
     let item = dict.get_item("key")?.expect("key not found in dict");
     let string = item.to_string();
     callback.call0()?;