ci: test linting only with python 3.9

Remove all python versions but the 3.9 from all liting commands.
The reason is that there's no need to run pylint on multiple python
versions if we check for the same issues.

Signed-off-by: Andrea Cervesato <[email protected]>

Author: acerv

.github/workflows/linting.yml

@@ -10,16 +10,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: [
-          "3.6",
-          "3.7",
-          "3.8",
-          "3.9",
-          "3.10",
-          "3.11",
-          "3.12",
-          "3.13"
-        ]
+        python-version: ["3.9"]
 
     steps:
     - name: Show OS