Fix \s deprecation warning

[Zsailer]

Fixes #591

[codecov-commenter]

Codecov Report

Merging #600 (aea4ae7) into master (8b9a3f7) will increase coverage by 0.06%.
The diff coverage is 100.00%.

❗ Current head aea4ae7 differs from pull request most recent head b152441. Consider uploading reports for the commit b152441 to get more accurate results

@@            Coverage Diff             @@
##           master     #600      +/-   ##
==========================================
+ Coverage   77.33%   77.40%   +0.06%     
==========================================
  Files         110      110              
  Lines       10251    10250       -1     
  Branches     1258     1257       -1     
==========================================
+ Hits         7928     7934       +6     
+ Misses       1921     1918       -3     
+ Partials      402      398       -4     

Impacted Files	Coverage Δ
jupyter_server/auth/login.py	68.93% <100.00%> (ø)
...upyter_server/tests/services/nbconvert/test_api.py	100.00% <0.00%> (ø)
jupyter_server/serverapp.py	65.53% <0.00%> (+0.18%)	⬆️
...ter_server/tests/services/contents/test_manager.py	96.95% <0.00%> (+0.23%)	⬆️
jupyter_server/services/contents/filemanager.py	70.55% <0.00%> (+0.81%)	⬆️

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