Re-add phpunit ^9.0 for PHP8

Author: Alex Westergaard

composer.json

@@ -46,7 +46,7 @@
         "guzzlehttp/guzzle": "^7.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^10.0"
+        "phpunit/phpunit": "^9.0|^10.0"
     },
     "minimum-stability": "stable",
     "prefer-stable": true