update phpunit to 9

Author: Dominik Meyer

composer.json

@@ -15,7 +15,7 @@
     }
   },
   "require-dev": {
-    "phpunit/phpunit": "^8",
+    "phpunit/phpunit": "^9",
     "squizlabs/php_codesniffer": "3.*"
   },
   "require": {