Allow 2.0 of Guzzle PSR-7 (#54)

Author: Nyholm

composer.json

@@ -18,7 +18,7 @@
         "php": "^7.1 || ^8.0",
         "phpunit/phpunit": "^7.5.20 || ^8.5.8 || ^9.3.11",
         "php-http/message": "^1.0 || ^2.0",
-        "guzzlehttp/psr7": "^1.7",
+        "guzzlehttp/psr7": "^1.7 || ^2.0",
         "th3n3rd/cartesian-product": "^0.3"
     },
     "suggest": {