Universal builds on Mac and setup.py clean-up

[jesterKing]

• Ensure building of universal binary of library for MacOS
• Use modern path and file idioms
• Don't os.chdir, that can leave host in unknown state
• Use subprocess for safer control, use cwd parameter to it

[fraguada]

Everything builds: https://github.com/mcneel/rhino3dm/actions/runs/9481884002