OpenBSD IPv6 sockets are always IPv6 only, and the option is readonly. Do not try to set it at all.

Author: Niklas Hallqvist

tests/socket.rs

@@ -1201,7 +1201,12 @@ test!(IPv4 recv_tos, set_recv_tos(true));
 test!(IPv4 broadcast, set_broadcast(true));
 
 test!(IPv6 unicast_hops_v6, set_unicast_hops_v6(20));
-#[cfg(not(any(windows, target_os = "dragonfly", target_os = "freebsd")))]
+#[cfg(not(any(
+    windows,
+    target_os = "dragonfly",
+    target_os = "freebsd",
+    target_os = "openbsd"
+)))]
 test!(IPv6 only_v6, set_only_v6(true));
 // IPv6 socket are already IPv6 only on FreeBSD and Windows.
 #[cfg(any(windows, target_os = "freebsd"))]