Hello,
You were able to install on stretch?
I was attempting to do so, but the python3-django-auth-ldap package wasn't available on anything but sid.
In order to install on Stretch, you should activate the stretc backports as explained here https://validation.linaro.org/static/docs/v2/installing_on_debian.html#insta...
So: 1/ adding "deb http://deb.debian.org/debian stretch-backports main" to apt source 2/ apt update 3/ apt -t stretch-backports install lava-server
Regards