Fix OS-specific path handling in setup.py

Attempting to install HTTPretty on Windows (using either ``pip`` or ``easy_install`` or ``python setup.py install``) fails with the following error:

```
WARNING: '\\.httpretty' not a valid package name; please use only.-separated package names in setup.py

error: package directory '\httpretty' does not exist
```

This patch fixes an OS-specific remnant of the ``get_packages`` magic in ``setup.py`` that's causing the error.  (Although really I'd recommend getting rid of it entirely since it's completely unnecessary for this project.)