Add PHP 8 support

Author: Anton Komarev

composer.json

@@ -39,7 +39,7 @@
         "docs": "https://github.com/cybercog/php-contracts-weblog/wiki"
     },
     "require": {
-        "php": "^7.1.3"
+        "php": "^7.1.3|^8.0"
     },
     "autoload": {
         "psr-4": {