Remove leading slash in notebook path

[mwouts]

With this PR we add path = path.strip('/') to both the get and save methods of Jupytext's contents manager, to match the reference implementation in notebook/services/contents/filemanager.py, cf. here:

https://github.com/jupyter/notebook/blob/43df5af2b614088b4b297fae90a70b6505b9bf84/notebook/services/contents/filemanager.py#L417

and here:

https://github.com/jupyter/notebook/blob/43df5af2b614088b4b297fae90a70b6505b9bf84/notebook/services/contents/filemanager.py#L451

This should fix #444 .

[codecov]

Codecov Report

Merging #445 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #445      +/-   ##
==========================================
+ Coverage   98.91%   98.91%   +<.01%     
==========================================
  Files          77       77              
  Lines        7563     7565       +2     
==========================================
+ Hits         7481     7483       +2     
  Misses         82       82

Impacted Files	Coverage Δ
tests/test_paired_paths.py	100% <100%> (ø)	⬆️
jupytext/version.py	100% <100%> (ø)	⬆️
jupytext/contentsmanager.py	98.51% <100%> (+0.01%)	⬆️

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