AST-1584 Make isort detect dp3 as first-party module
Depending on the directory name of the directory containing DP3 sources, isort behaves differently:
- If the directory is called 'dp3', isort thinks dp3 is part of the current python project.
- If the directory is called 'DP3', isort thinks dp3 is some external third-party module.
Adding an extra configuration file ensures that the behavior for the dp3 module is always the same. See also https://pycqa.github.io/isort/docs/configuration/options.html#known-first-party