Run CI tests on Debian Bullseye

Production now runs on Bullseye

.gitlab-ci.yml

 linter:
-  image: registry.git.fsmpi.rwth-aachen.de/videoaginfra/testenvs/stretch
+  image: registry.git.fsmpi.rwth-aachen.de/videoaginfra/testenvs/bullseye
   stage: test
   script:
   - uname -a
@@ -11,7 +11,7 @@ linter:
       - pylint.txt
 
 unittest: &unittest
-  image: registry.git.fsmpi.rwth-aachen.de/videoaginfra/testenvs/stretch
+  image: registry.git.fsmpi.rwth-aachen.de/videoaginfra/testenvs/bullseye
   stage: test
   script:
   - uname -a
@@ -25,27 +25,3 @@ unittest: &unittest
       - htmlcov/*
       - .coverage
       - report.txt
-
-# Buster is not supported for now
-#
-#unittest_buster:
-#  <<: *unittest
-#  image: debian:buster
-#
-#livetest:
-#  image: debian:stretch
-#  stage: test
-#  script:
-#  - apt update
-#  - apt install -y python3
-#  - python3 -V
-#  - uname -a
-#  - apt install -y python3-requests
-#    - ./tests/urlcheck_sinf.py
-#
-#deploy_staging:
-#  image: archlinux/base
-#  stage: deploy
-#  script:
-#  - pacman --noconfirm -Sy ansible git
-#