always use relative paths in data_files

instead of absolute paths for sources

seems to have started failing on Windows

setup.py

@@ -46,10 +46,9 @@ def get_data_files():
     """Get data files in share/jupyter"""
 
     data_files = []
-    ntrim = len(here + os.path.sep)
-
     for (d, dirs, filenames) in os.walk(share_jupyterhub):
-        data_files.append((d[ntrim:], [pjoin(d, f) for f in filenames]))
+        rel_d = os.path.relpath(d, here)
+        data_files.append((rel_d, [os.path.join(rel_d, f) for f in filenames]))
     return data_files