missing isort dependency

including the demessaging dependency alone to create a backend module results in the following exception:

Traceback (most recent call last):
  File "xai4flood_backend_module.py", line 54, in <module>
    main(topic="xai4flood", max_workers=4, queue_size=4)
  File "/home/eggert/workspace/python/ai4flood/backend/env/lib/python3.8/site-packages/demessaging/backend/__init__.py", line 56, in main
    Model = BackendModule.create_model(module_name, config=config)
  File "/home/eggert/workspace/python/ai4flood/backend/env/lib/python3.8/site-packages/demessaging/backend/module.py", line 259, in create_model
    config.imports += "\n" + utils.get_module_imports(module)
  File "/home/eggert/workspace/python/ai4flood/backend/env/lib/python3.8/site-packages/demessaging/backend/utils.py", line 188, in get_module_imports
    from isort.api import find_imports_in_code
ModuleNotFoundError: No module named 'isort'