ci: Test in a bookworm container

In order to be able to provide all functions of the DBus during testing,
the CI uses the Python 3.11 version in a Bookworm container for
testing. This is the version that is pre-installed in Debian Bookworm.

Signed-off-by: Sven Sager <s.sager@kunbus.com>

.gitlab-ci.yml

@@ -24,7 +24,7 @@ include:
 
 run_tests:
   stage: test
-  image: python:3
+  image: python:3.11-bookworm
   script:
     - apt-get update
     - apt-get -y install dbus libgirepository1.0-dev