Creating VPK from directory doesn't work on Windows

[JCRPaquin]

Looks like file paths are tripping up Windows. Specifically, I couldn't create a VPK given a directory on Windows under Python 3.6.

I've fixed it locally using pathlib but haven't tested on a *nix platform yet; will push these changes to a fork soon.

Using pathlib may cause issues with Python 2; I'm not sure how much this upsets you.

[rossengeorgiev]

Any stracktrace? Steps to reproduce?

[JCRPaquin]

Python 3.6.3, Windows 10

To reproduce:

1. Make a root folder
2. Make a subfolder
3. Add a file to the subfolder
4. Run vpk -c <root folder name> <vpk name>

The process should fail with an IOError indicating that no such file was found. No stack trace is output.

[rossengeorgiev]

Cool, I reproduced the issue.