Update to PHP 8

test/Unit/Adapter/FilesystemAdapterTest.php

@@ -121,7 +121,7 @@ final class FilesystemAdapterTest extends AbstractTestCase
         $adapter->delete('a/b');
     }
 
-    protected function tearDown()
+    protected function tearDown(): void
     {
         parent::tearDown();
         $files = new RecursiveIteratorIterator(

test/Unit/Adapter/NamespaceAdapterTest.php

@@ -15,7 +15,7 @@ final class NamespaceAdapterTest extends AbstractTestCase
      */
     private $adapter;
 
-    public function setUp()
+    public function setUp(): void
     {
         $this->adapter = new MemoryAdapter();
     }

test/Unit/Serializer/JsonSerializerTest.php

@@ -13,9 +13,7 @@ final class JsonSerializerTest extends AbstractTestCase
         $string = 'myString';
         $array = [0 => '1', 1 => '2', 2 => '3'];
 
-        $this->assertInternalType('string', $serializer->serialize($array));
         $this->assertEquals($string, $serializer->serialize($string));
-
         $this->assertEquals($array, $serializer->deserialize($serializer->serialize($array)));
     }