Don't block the event loop when exporting with nbconvert

[davidbrochart]

No description provided.

[codecov-commenter]

Codecov Report

Merging #655 (e0171ae) into main (db46446) will increase coverage by 0.05%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##             main     #655      +/-   ##
==========================================
+ Coverage   77.88%   77.94%   +0.05%     
==========================================
  Files         110      110              
  Lines       10405    10406       +1     
  Branches     1400     1400              
==========================================
+ Hits         8104     8111       +7     
+ Misses       1911     1907       -4     
+ Partials      390      388       -2     

Impacted Files	Coverage Δ
jupyter_server/nbconvert/handlers.py	25.00% <50.00%> (+0.72%)	⬆️
jupyter_server/services/kernels/handlers.py	62.71% <0.00%> (+1.45%)	⬆️

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[blink1073]

Thanks!