Make Serializable implementation internal and final

Author: derrabus

Authentication/Token/NullToken.php

@@ -103,6 +103,9 @@ public function __unserialize(array $data): void
 
     /**
      * @return string
+     *
+     * @internal in 5.3
+     * @final in 5.3
      */
     public function serialize()
     {
@@ -111,6 +114,9 @@ public function serialize()
 
     /**
      * @return void
+     *
+     * @internal in 5.3
+     * @final in 5.3
      */
     public function unserialize($serialized)
     {