Start testing on PHP 8.1 (#164)

Author: sanmai

.github/workflows/tests.yaml

@@ -16,7 +16,7 @@ jobs:
 
     strategy:
       matrix:
-        php-version: ['7.3', '7.4', '8.0']
+        php-version: ['7.3', '7.4', '8.0', '8.1']
         include:
           - { php-version: '7.3', dependencies: '--prefer-lowest', legend: 'with lowest dependencies' }