Add full support for PHP 8.0

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

tests/Spout/Reader/XLSX/Manager/SharedStringsManagerTest.php

@@ -25,7 +25,7 @@ class SharedStringsManagerTest extends TestCase
     /**
      * @return void
      */
-    public function setUp()
+    public function setUp(): void
     {
         $this->sharedStringsManager = null;
     }
@@ -33,7 +33,7 @@ class SharedStringsManagerTest extends TestCase
     /**
      * @return void
      */
-    public function tearDown()
+    public function tearDown(): void
     {
         if ($this->sharedStringsManager !== null) {
             $this->sharedStringsManager->cleanup();

tests/Spout/Reader/XLSX/ReaderTest.php

@@ -539,6 +539,10 @@ class ReaderTest extends TestCase
      */
     public function testReadShouldBeProtectedAgainstQuadraticBlowupAttack()
     {
+        if (function_exists('xdebug_code_coverage_started') && xdebug_code_coverage_started()) {
+            $this->markTestSkipped('test not compatible with code coverage');
+        }
+
         $startTime = microtime(true);
 
         $this->getAllRowsForFile('attack_quadratic_blowup.xlsx');