composer.json: relax phpunit version requirment to allow testing on PHP 8.0

Author: zozlak

composer.json

@@ -32,7 +32,7 @@
         }
     },
     "require-dev": {
-        "phpunit/phpunit": "^10",
+        "phpunit/phpunit": "*",
         "phpstan/phpstan": "^1",
         "php-coveralls/php-coveralls": "^2.4",
         "sweetrdf/quick-rdf": "^2",