chore: allow PHPUnit ^9

Signed-off-by: Jack Cherng <jfcherng@gmail.com>

Author: jfcherng

composer.json

@@ -36,7 +36,7 @@
     "require-dev": {
         "friendsofphp/php-cs-fixer": "^2.15",
         "phan/phan": "^2.2",
-        "phpunit/phpunit": "^7.2 || ^8.2",
+        "phpunit/phpunit": "^7.2 || ^8.2 || ^9",
         "squizlabs/php_codesniffer": "^3.5"
     },
     "config": {