Fix duplicity to Just Work

This way its bin/*:

- use system python, not one I arbitrarily installed with say pyenv
- have a proper sys.path/PYTHONPATH set

TODO:

- additional sys.path is kinda hardcoded
- python executable is called hardcoded too instead of using brew's
  python magic. In this case homebrew tries too hard to pick my python
  instead of the system one.