ci: allow failure for Debian bullseye (incompatible changes)

There are incompatible changes in some Python modules that make the
tests fail. As it is a new test, allow the failure until this is
fixed.

.gitlab-ci.yml

@@ -33,6 +33,7 @@ tests bullseye python3:
     paths:
       - coverage
       - output
+  allow_failure: true
 
 flake8:
   <<: *template