update jpserver_extensions trait to work with traitlets 5.x

[Zsailer]

Fixes #606

[codecov-commenter]

Codecov Report

Merging #610 (fb3ec89) into master (a1b013e) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #610      +/-   ##
==========================================
- Coverage   77.41%   77.39%   -0.02%     
==========================================
  Files         110      110              
  Lines       10252    10252              
  Branches     1259     1259              
==========================================
- Hits         7937     7935       -2     
- Misses       1918     1919       +1     
- Partials      397      398       +1     

Impacted Files	Coverage Δ
jupyter_server/serverapp.py	65.47% <ø> (ø)
jupyter_server/tests/services/kernels/test_api.py	97.02% <0.00%> (-1.99%)	⬇️

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[mwakaba2]

Thanks Zach! I confirmed it's working!

$ jupyter server -ServerApp.jpserver_extensions "jupyterlab"=True
[I 2021-11-14 21:40:06.895 ServerApp] jupyterlab | extension was successfully linked.