Fix nbconvert handler run_sync()

[davidbrochart]

Fixes jupyterlab/jupyterlab#11885
Fixes jupyterlab/jupyterlab#11877

[codecov-commenter]

Codecov Report

Merging #667 (72e94b0) into main (923a828) will decrease coverage by 0.04%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##             main     #667      +/-   ##
==========================================
- Coverage   77.85%   77.81%   -0.05%     
==========================================
  Files         110      110              
  Lines       10415    10415              
  Branches     1401     1403       +2     
==========================================
- Hits         8109     8104       -5     
- Misses       1916     1922       +6     
+ Partials      390      389       -1     

Impacted Files	Coverage Δ
jupyter_server/nbconvert/handlers.py	25.00% <50.00%> (ø)
jupyter_server/services/kernels/kernelmanager.py	80.63% <0.00%> (-1.91%)	⬇️
jupyter_server/tests/services/kernels/test_api.py	94.92% <0.00%> (-1.45%)	⬇️
jupyter_server/utils.py	66.47% <0.00%> (+0.56%)	⬆️
jupyter_server/pytest_plugin.py	85.83% <0.00%> (+0.85%)	⬆️

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[Zsailer]

Thank you, @davidbrochart. The test failures are unrelated. They appear to be caused by jupyter/jupyter_client#732

[Zsailer]

Thank you, @davidbrochart. The test failures are unrelated. They appear to be caused by jupyter/jupyter_client#732