Fix pipeline

.gitlab-ci.yml

-image: python:3.8-buster
+image: python:3.9-bullseye
 
 variables:
   GIT_SUBMODULE_STRATEGY: recursive
@@ -7,6 +7,7 @@ variables:
 pages:
   stage: deploy
   before_script:
+  - apt update; apt install po4a
   - pip install -r requirements.txt
   script:
   - make publish; make publish