Slicing a github.PaginatedList.PaginatedList can produce an IndexError

[mpiraux]

Slicing a PaginatedList from PyGithub can produce an IndexError if the list is empty (while slicing an empty Python list do not). A few fixes can be envisaged:

1. Check if the list is not empty before slicing and iterating over it - Not pythonic
2. Monkey patch the PaginatedList class to properly re-implement slicing
3. Open an issue and propose a PR to PyGithub