Unfortunatly, this doesn’t seem to solve my problem 
I added
before_install: - export PYTHONPATH=$PYTHONPATH:$(pwd)
and checked the variable via echo. It looks good and includes the directory of my project. Also just invoking python manage.py test results in this issue …
I also tried to rollout the repo on another server to avoid bugs due local artifacts, but also on that server everything works fine.
Sorry, but any further ideas?