make path relative

[phaustin]

the validate button is failing for me on current master with error messages like:

FileNotFoundError: [Errno 2] No such file or directory: '/source/Assign_1'

this is because path is being set with '/' as the first character, making the os.path.join with the notebook_dir a noop. My fix using pathlib.relative_to works, but obviously it would be better to catch this at the initialization and make sure path is relative.

[jhamrick]

Hmm, I'm not able to reproduce this unfortunately. Do you know why the path is starting with a / in the first place? I would think that shouldn't happen... can you open a JavaScript console on the offending notebook and look at what Jupyter.notebook.notebook_path is?

But I agree if this is possible we should check that the path is relative rather than absolute. However, we haven't quite yet dropped support for Python 2 (happening very soon though). Would you mind writing this without pathlib so that the python 2 tests will still pass?

[jabalazs]

I am having the same issue. The Jupyter.notebook.notebook_path in my case is /source/assignment_3/assignment_3.ipynb.

Steps to replicate:

1. Create a new conda environment:
   conda create -n nbgrader python=3.7
2. Install nbgrader:
   conda install nbgrader -c conda-forge
   At this point, the validate button still works.
3. Install jupytext:
   conda install jupytext -c conda-forge
   The validate button does not work anymore, and displays the error referenced above.