crontab command not working with flop_admin

When using the script flop_admin for executing the crontab command, an error is raised with the following message : ERROR: Can't find your manage.py - please define settings.CRONTAB_DJANGO_MANAGE_PATH puis AttributeError: 'Settings' object has no attribute 'DJANGO_MANAGE_PATH'.

The reason is that in production, the manage.py script is not available so we must check if flop_admin is available in the venv and set the variable with the correct path.