update tests

Author: Awilum

.github/workflows/tests.yml

@@ -9,7 +9,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        php: ['7.4', '8.0']
+        php: ['7.4', '8.0', '8.1']
         dependency-version: [prefer-lowest, prefer-stable]
 
     steps: