Check for both os.path.sep and os.path.altsep

When normalizing paths, we want to handle the following cases:

- Someone is using a Windows-style path on Windows
- Someone is using a Unix style path on Unix
- Someone is using a Unix style path on Windows

os.path.sep will handle the native directory separator character while
os.path.altsep (when set) will handle alternate separators. Further,
os.path.abspath does the right thing on Windows when handed a Unix-style
path.

Related to #175