Add full support for PHP 8.0

Unfortunately due to PHPUnit 8.5 dependency
this also drops support for PHP 7.1

tests/Spout/Common/Entity/RowTest.php

@@ -122,6 +122,6 @@ class RowTest extends \PHPUnit\Framework\TestCase
             ->setStyle($this->getStyleMock())
             ->setCells([]);
 
-        $this->assertInternalType('object', $row);
+        $this->assertInstanceOf(Row::class, $row);
     }
 }

tests/Spout/Common/Helper/FileSystemHelperTest.php

@@ -16,7 +16,7 @@ class FileSystemHelperTest extends TestCase
     /**
      * @return void
      */
-    public function setUp()
+    public function setUp() : void
     {
         $baseFolder = \sys_get_temp_dir();
         $this->fileSystemHelper = new FileSystemHelper($baseFolder);

tests/Spout/Common/Manager/OptionsManagerTest.php

@@ -14,7 +14,7 @@ class OptionsManagerTest extends TestCase
      */
     protected $optionsManager;
 
-    protected function setUp()
+    protected function setUp(): void
     {
         $this->optionsManager = new class() extends OptionsManagerAbstract {
             protected function getSupportedOptions()