chore(deps): update dependency phpunit/phpunit to v9 (#81)

Co-authored-by: WhiteSource Renovate <renovatebot@gmail.com>

composer.json

@@ -15,7 +15,7 @@
     },
     "require-dev": {
         "friendsofphp/php-cs-fixer": "^2.16",
-        "phpunit/phpunit": "^8.0.0"
+        "phpunit/phpunit": "^9.0.0"
     },
     "suggest": {
         "ext-mbstring": "If ext-mbstring is not available you MUST install symfony/polyfill-mbstring"