Add full support for PHP 8.0

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

tests/Spout/Writer/XLSX/WriterWithStyleTest.php

@@ -32,7 +32,7 @@ class WriterWithStyleTest extends TestCase
     /**
      * @return void
      */
-    public function setUp()
+    public function setUp(): void
     {
         $this->defaultStyle = (new StyleBuilder())->build();
     }