Airflow-pentaho-plugin

- Fixes Log Level Problem [9](https://github.com/damavis/airflow-pentaho-plugin/issues/9)

- Advanced statuses handling. (by [IM-SIM](https://github.com/IM-SIM))

- Backported to Airflow version 1.10.x.
- Parameters format for kettle Operators fixed.
- Testing on pre-commit hook added.
- xmldtodict bumped from 0.10.0 to 0.12.0.
- Pypi classifiers fixed.
- Style fixes.
- Add CI/CD testing for python 3.6, 3.7, 3.8 and 3.9.
- Pinning version of SQLAchemy<1.4,>=1.3.18.

Deprecations

- Operators have been reorganized into 2 modules, and the old modules had been
marked as deprecated and will be removed in the future.

- Kettle
- `airflow_pentaho.hooks.PentahoHook -> airflow_pentaho.hooks.kettle`
- `airflow_pentaho.operators.PDIBaseOperator -> airflow_pentaho.operators.kettle`
- `airflow_pentaho.operators.KitchenOperator -> airflow_pentaho.operators.kettle`
- `airflow_pentaho.operators.PanOperator -> airflow_pentaho.operators.kettle`
- Carte
- `airflow_pentaho.hooks.PentahoCarteHook -> airflow_pentaho.hooks.carte`
- `airflow_pentaho.operators.CarteBaseOperator -> airflow_pentaho.operators.carte`
- `airflow_pentaho.operators.CarteJobOperator -> airflow_pentaho.operators.carte`
- `airflow_pentaho.operators.CarteTransOperator -> airflow_pentaho.operators.carte`

Fixes

- Allow users to choose http:// or https:// for Carte host.
- Other minor fixes.