Add "import os" in setup.py

[Arkelis]

When adding hy as a git depedency with Poetry, it tries to read setup.py and crashes because os is not imported

[Kodiologist]

Now I wonder why setup.py works in the first place. Doesn't pip install execute it? So shouldn't it throw NameError when it hits os? Man, Python package management is mysterious.

Anyway, this is a very small change and seems fine to me. Thanks for catching it.