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

src/Spout/Reader/Wrapper/XMLReader.php

@@ -11,7 +11,7 @@ class XMLReader extends \XMLReader
 {
     use XMLInternalErrorsHelper;
 
-    const ZIP_WRAPPER = 'zip://';
+    public const ZIP_WRAPPER = 'zip://';
 
     /**
      * Opens the XML Reader to read a file located inside a ZIP file.