Paired notebooks dont work when notebook name contains a dot

[mwouts]

Reported by @mppaskov at #70

Not sure if this is related but I had a notebook named 1.Some Notebook.ipynb and it was not exporting automatically to .py. Even after manually exporting the notebook with the command line it would not sync. Was able to fix this by renaming the notebook Some Notebook.ipynb.

The bug occurs because the extension of the initial notebook is recognized as .Some Notebook.ipynb, which is not associated to .py (unless the user sets
"jupytext_formats":"Some Notebook.ipynb,py" , which we clearly do not want!)

[mwouts]

This is caused by mwouts/nbsrc#5: in case the user has a script with the same name as the notebook, he may want to save the text of the notebook to a notebook.nb.py file with extension nb.py. Possibly we want to remove that possibility to make practice simpler!