fix: Hopefully fix issues with PHP 8.1.

Author: drupol

src/Node/Node.php

@@ -10,6 +10,7 @@
 namespace loophp\phptree\Node;
 
 use InvalidArgumentException;
+use ReturnTypeWillChange;
 use Traversable;
 
 use function array_key_exists;
@@ -133,9 +134,9 @@ public function getAncestors(): Traversable
     }
 
     /**
-     * @return Traversable<\loophp\phptree\Node\NodeInterface>
+     * @return Traversable<NodeInterface>
      */
-    public function getIterator()
+    public function getIterator(): Traversable
     {
         yield from $this->all();
     }
@@ -204,6 +205,7 @@ public function level(int $level): Traversable
      *
      * @return bool
      */
+    #[ReturnTypeWillChange]
     public function offsetExists($offset)
     {
         return array_key_exists($offset, $this->children);
@@ -214,6 +216,7 @@ public function offsetExists($offset)
      *
      * @return NodeInterface
      */
+    #[ReturnTypeWillChange]
     public function offsetGet($offset)
     {
         return $this->children[$offset];
@@ -239,6 +242,7 @@ public function offsetSet($offset, $value): void
      *
      * @return void
      */
+    #[ReturnTypeWillChange]
     public function offsetUnset($offset)
     {
         unset($this->children[$offset]);