Add phpstan until level 7

tests/Spout/Reader/CSV/SheetTest.php

@@ -3,6 +3,7 @@
 namespace Box\Spout\Reader\CSV;
 
 use Box\Spout\Reader\Common\Creator\ReaderEntityFactory;
+use Box\Spout\Reader\SheetInterface;
 use Box\Spout\TestUsingResource;
 use PHPUnit\Framework\TestCase;
 
@@ -27,7 +28,7 @@ class SheetTest extends TestCase
 
     /**
      * @param string $fileName
-     * @return Sheet
+     * @return SheetInterface
      */
     private function openFileAndReturnSheet($fileName)
     {