Makefile adjusts
Makefile adjusts

- Use PYTHON environment var instead of `python`. Default is Python 3
- Use PyTest for the tests (PYTEST controls the executable v3 default)
- Avoid errors on `clean-test`

hildogjr/KiCost,xesscorp/KiCost,xesscorp/KiCost,hildogjr/KiCost,xesscorp/KiCost,hildogjr/KiCost