Avoid dependency on NBConvert versions for REST API test

[Zsailer]

Relaxes one of the NBConvert API tests, so it's not dependent on NBConvert versions and only tests the REST API.

[Zsailer]

Should fix the test failures in #599

[codecov-commenter]

Codecov Report

Merging #601 (968a637) into master (29be9c6) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #601      +/-   ##
==========================================
+ Coverage   77.34%   77.40%   +0.06%     
==========================================
  Files         110      110              
  Lines       10250    10251       +1     
  Branches     1257     1258       +1     
==========================================
+ Hits         7928     7935       +7     
+ Misses       1926     1918       -8     
- Partials      396      398       +2     

Impacted Files	Coverage Δ
...upyter_server/tests/services/nbconvert/test_api.py	100.00% <100.00%> (ø)
jupyter_server/services/kernels/kernelmanager.py	81.84% <0.00%> (+2.05%)	⬆️

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[kevin-bates]

Looks good and makes sense (to relax the checks) - thank you!