Do not mix the use of `os`, `pathlib` and `shutil`. Try to use `pathlib` where possible.

[TzuChieh]

pathlib is the modern solution for path/filesystem related operations, and has a cleaner and more consistent interface IMHO.

[TzuChieh]

• [ ] Cleans up mixed use in PhotonBlend
• [ ] Cleans up mixed use in setup script
• [ ] Find other mixed uses and clean them up