ci: use python 3.11

.gitlab-ci-extended.yml

 py3:
-  image: python:3.9-bullseye
+  image: python:3.11
   before_script:
     - pip install tox

.gitlab-ci.yml

 ---
 default:
-  image: python:3.7
+  image: python:3.11
 
 include:
   - project: "open-source/gitlab-ci-templates"