phpunit 9 requires php >= 7.3

.github/workflows/tests.yml

@@ -15,7 +15,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        php-versions: ['5.6', '7.0', '7.1', '7.2', '7.3', '7.4', '8.0']
+        php-versions: ['7.3', '7.4', '8.0']
     name: PHP ${{ matrix.php-versions }} unit tests on ${{ matrix.operating-system }}
     steps:
     - name: Checkout