Support Different OS File Separators 

Use the `os.sep` constant instead of hard coded `/` to support Windows execution.  
**From:** `prg = ./hello.py`
**To:** `prg = '.' + os.sep + 'hello.py'`

[Example of Issue](https://github.com/kyclark/tiny_python_projects/blob/f9151ba80c78301b62c724d9ed9c2a0c493c021a/01_hello/test.py#L7)

[OS Docs](https://docs.python.org/3/library/os.html)