add php 8.1 unit tests

.github/workflows/tests.yml

@@ -17,7 +17,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        php-versions: ['7.3', '7.4', '8.0']
+        php-versions: ['7.3', '7.4', '8.0', '8.1']
     name: PHP ${{ matrix.php-versions }} unit tests on ${{ matrix.operating-system }}
     env:
       extensions: gd, sqlite3