Added support for PHP 8.1

Author: GrahamCampbell

.github/workflows/tests.yml

@@ -11,7 +11,7 @@ jobs:
 
     strategy:
       matrix:
-        php: ['7.2', '7.3', '7.4', '8.0']
+        php: ['7.2', '7.3', '7.4', '8.0', '8.1']
 
     steps:
       - name: Checkout Code

README.md

@@ -23,7 +23,7 @@ Check out the [change log](CHANGELOG.md), [releases](https://github.com/GitLabPH
 
 ## Installation
 
-This version supports [PHP](https://php.net) 7.2-8.0. To get started, simply require the project using [Composer](https://getcomposer.org). You will also need to install packages that "provide" [`psr/http-client-implementation`](https://packagist.org/providers/psr/http-client-implementation) and [`psr/http-factory-implementation`](https://packagist.org/providers/psr/http-factory-implementation).
+This version supports [PHP](https://php.net) 7.2-8.1. To get started, simply require the project using [Composer](https://getcomposer.org). You will also need to install packages that "provide" [`psr/http-client-implementation`](https://packagist.org/providers/psr/http-client-implementation) and [`psr/http-factory-implementation`](https://packagist.org/providers/psr/http-factory-implementation).
 
 ### Standard Installation