Added tests for PHP 8.3.

Author: sunaoka

.github/workflows/test.yml

@@ -16,7 +16,7 @@ jobs:
         php: [
           '5.3', '5.4', '5.5', '5.6',
           '7.0', '7.1', '7.2', '7.3', '7.4',
-          '8.0', '8.1', '8.2'
+          '8.0', '8.1', '8.2', '8.3'
         ]
 
     name: PHP ${{ matrix.php }}