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/Common/Manager/Style/StyleMergerTest.php

@@ -18,7 +18,7 @@ class StyleMergerTest extends TestCase
     /**
      * @return void
      */
-    public function setUp()
+    public function setUp(): void
     {
         $this->styleMerger = new StyleMerger();
     }

tests/Spout/Writer/Common/Manager/Style/StyleRegistryTest.php

@@ -20,7 +20,7 @@ class StyleRegistryTest extends TestCase
     /**
      * @return void
      */
-    public function setUp()
+    public function setUp(): void
     {
         $this->defaultStyle = (new StyleBuilder())->build();
         $this->styleRegistry = new StyleRegistry($this->defaultStyle);