ci: set python-version for ruff job

Author: alexfikl

.github/workflows/ci.yml

@@ -24,8 +24,9 @@ jobs:
         runs-on: ubuntu-latest
         steps:
         -   uses: actions/checkout@v5
-        -
-            uses: actions/setup-python@v6
+        -   uses: actions/setup-python@v6
+            with:
+                python-version: '3.x'
         -   name: "Main Script"
             run: |
                 pip install ruff