Remove never_type feature gate

The stabilization for this feature has landed in nightly.

test_suite/tests/test_ser.rs

@@ -1,5 +1,4 @@
 #![allow(clippy::unreadable_literal)]
-#![cfg_attr(feature = "unstable", feature(never_type))]
 
 use std::cell::RefCell;
 use std::collections::{BTreeMap, BTreeSet, HashMap, HashSet};