Flake8-future-import

* Fix error reported with flake8 version 5

- Do not ignore imports which are present and have been added after the minimum version
- Ignore imports which became mandatory with the minimum version

- Add two older `future` imports
- Issue an error when a future import does not exist
- Define which is the oldest Python version to be supported so that already mandatory features can be ignored and not yet supported features default to forbidden (ignoring the lower error code).
- Use return code of 1 if errors occurred