Substitution for os.sep

[jayvdb]

There is {:} for os.pathsep , but none for os.sep (/ vs \ on Windows). I propose {/} for this.

This is probably because most of the time / works ok on Windows, but that is not universal.

However the lack of os.sep support also impacts the ability of config tests to do meaningful tests of paths that are cross platform without horrible contortions for Windows.