Do not move stuff around in setup.py

Your current approach of moving stuff around in `setup.py` is very unclean:

Even without passing an actual action (`install`, `sdist` etc.) the script will copy stuff around. Also, you will not be able to build source packages for Windows when you are on any other operating system.

My solution: Always ship the DLLs and make the distinction in your actual module, not in `setup.py`.