Fix nbconvert handler

[davidbrochart]

It looks like the call to nbconvert's get_exporter function is not thread-safe:

jupyter_server/jupyter_server/services/nbconvert/handlers.py

Line 23 in 450010e

exporter_class = await run_sync(base.get_exporter, exporter_name)

And I can see that JupyterLab makes multiple concurrent requests to /api/nbconvert. This PR works around that issue using a lock.

Fixes jupyterlab/jupyterlab#10442

[codecov-commenter]

Codecov Report

Merging #545 (1baa5ba) into master (450010e) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #545      +/-   ##
==========================================
- Coverage   76.33%   76.32%   -0.02%     
==========================================
  Files         110      110              
  Lines        9816     9819       +3     
  Branches     1067     1067              
==========================================
+ Hits         7493     7494       +1     
- Misses       1943     1944       +1     
- Partials      380      381       +1     

Impacted Files	Coverage Δ
jupyter_server/services/nbconvert/handlers.py	83.33% <100.00%> (+2.38%)	⬆️
jupyter_server/services/kernels/kernelmanager.py	80.07% <0.00%> (-0.73%)	⬇️

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[blink1073]

Thanks!