amending dev package constraint

compatibility with phpunit/php-timer can't be verified by tests if it isn't installed in CI

Author: SignpostMarv

composer.json

@@ -21,7 +21,7 @@
     "phpunit/php-timer": "^2.0||^3.0"
   },
   "require-dev": {
-    "phpunit/phpunit": "^7.0||^8.0||^9.0",
+    "phpunit/phpunit": "^7.5.20||^8.5||^9.0",
     "squizlabs/php_codesniffer": "^2.8.1||^3.5"
   },
   "autoload": {