Actually test against Ruby 4.0...

Author: fbacall

.github/workflows/tests.yml

@@ -5,7 +5,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby: ['2.7', '3.0', '3.1', '3.2', '3.3', '3.4']
+        ruby: ['2.7', '3.0', '3.1', '3.2', '3.3', '3.4', '4.0']
       fail-fast: false
     steps:
       - name: Checkout