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/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()