Upgrade PHP and dev dependencies

/!\ Removed PHP 7.2 support /!\

- PHPUnit 8 => 9 (+ fix the tests)
- PHP-CS-Fixer 2 => 3 (+ fix the code)
- Introduced PHP stan

tests/Spout/Reader/CSV/SpoutTestStream.php

@@ -9,10 +9,10 @@ namespace Box\Spout\Reader\CSV;
  */
 class SpoutTestStream
 {
-    const CLASS_NAME = __CLASS__;
+    public const CLASS_NAME = __CLASS__;
 
-    const PATH_TO_CSV_RESOURCES = 'tests/resources/csv/';
-    const CSV_EXTENSION = '.csv';
+    public const PATH_TO_CSV_RESOURCES = 'tests/resources/csv/';
+    public const CSV_EXTENSION = '.csv';
 
     /** @var int */
     private $position;