- formatted `kpi_view.py` using black and isort
- fixed some type errors
    - check the return value of `request.get_json()` is None before
      using it
    - variable may be unbound - moved initialization to the top. see
      `ret = []`.
- fixed flake8 errors
    - ignored in cases where it cannot be prevented
- fixed flake8 warnings:
    - some variables were unused - removed their assignment in case of
      functions with side effects and removed the entire line in case of
      functions without side effects (`DataSource.get_by_id` for example)
- isort
    - many unused imports
    - imports position was wrong
    - flask imports require it to be imported from a specific submodule
- added docstrings
- replaced prints with logger

When the error is on the client, the status code should be 400

…ints