Update PHPUnit from 8.x to 9.x

Author: forevermatt

composer.json

@@ -28,7 +28,7 @@
         "guzzlehttp/guzzle": "^6.2"
     },
     "require-dev": {
-        "phpunit/phpunit": "^8.5",
+        "phpunit/phpunit": "^9.5",
         "squizlabs/php_codesniffer": "~2.6",
         "friendsofphp/php-cs-fixer": "^2.0"
     },