Fix filename to open in notebookapp.py after #4260 and #4265

[slel]

This also needs to be backported to the 5.7.x branch and should fix #4303.

[takluyver]

That wasn't a mistake, open() on Python 3 takes a file descriptor, and closes it when it's done.

We should use io.open() for Python 2, which is effectively the Python 3 open() function, and accepts a file descriptor. Can you submit a PR to do this against the 5.7.x branch (master has already dropped Python 2 support, so we don't need any change there).

[slel]

Thanks for clarifying. I'll open a PR against the 5.7.x branch
changing with open to with io.open at line 1742 of the file
notebook/notebookapp.py.

What about the occurrence at line 1674, does that one stay as is?

https://github.com/jupyter/notebook/blob/5.7.x/notebook/notebookapp.py#L1742
https://github.com/jupyter/notebook/blob/5.7.x/notebook/notebookapp.py#L1674

[takluyver]

Thanks! Yes, I think they should both use io.open(..., encoding='utf-8').

[slel]

See #4348.