Fix jupyter_client warning

[martinRenou]

Fix the jupyter_client warning:

/home/martin/miniconda3/envs/xeus-python/lib/python3.9/json/encoder.py:257: 
UserWarning: date_default is deprecated since jupyter_client 7.0.0. Use jupyter_client.jsonutil.json_default.

[codecov-commenter]

Codecov Report

Merging #581 (cb37234) into master (49d5772) will decrease coverage by 0.05%.
The diff coverage is 67.74%.

@@            Coverage Diff             @@
##           master     #581      +/-   ##
==========================================
- Coverage   77.46%   77.41%   -0.06%     
==========================================
  Files         110      110              
  Lines       10184    10196      +12     
  Branches     1100     1100              
==========================================
+ Hits         7889     7893       +4     
- Misses       1907     1915       +8     
  Partials      388      388              

Impacted Files	Coverage Δ
jupyter_server/services/kernels/handlers.py	61.55% <60.00%> (-0.23%)	⬇️
jupyter_server/base/zmqhandlers.py	60.00% <66.66%> (-0.55%)	⬇️
jupyter_server/services/contents/handlers.py	83.63% <71.42%> (-0.94%)	⬇️
jupyter_server/services/sessions/handlers.py	73.14% <75.00%> (-1.14%)	⬇️

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[blink1073]

Thank you!