Use Traversable return type for getIterator()

Author: thekid

src/test/php/lang/ast/syntax/php/unittest/IsOperatorTest.class.php

@@ -130,7 +130,7 @@ public function run() {
   #[Test]
   public function is_native_iterable_type() {
     $r= $this->run('class <T> implements \IteratorAggregate {
-      public function getIterator() {
+      public function getIterator(): \Traversable {
         yield 1;
       }