make installable in PHP 8

composer.json

@@ -21,9 +21,9 @@
       "GPL-3.0-or-later"
     ],
     "require": {
-        "php": "^7.0",
+        "php": "^7.0 || ^8.0",
         "beberlei/assert": "^2.9.9",
-        "guzzlehttp/guzzle": "~6.2",
+        "guzzlehttp/guzzle": "~6.2 || ^7.4",
         "psr/http-message": "~1.0",
         "symfony/options-resolver": "^4.4.37",
         "phlib/sms-length": "^1.1.0 || ^2.0.0",