Add compat for openapi-core 0.15

[blink1073]

cf https://github.com/p1c2u/openapi-core/pull/393/files

[codecov-commenter]

Codecov Report

Merging #281 (9b37958) into main (0d9d027) will increase coverage by 0.02%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##             main     #281      +/-   ##
==========================================
+ Coverage   65.18%   65.20%   +0.02%     
==========================================
  Files          22       22              
  Lines        1735     1739       +4     
  Branches      296      296              
==========================================
+ Hits         1131     1134       +3     
- Misses        520      521       +1     
  Partials       84       84              

Impacted Files	Coverage Δ
jupyterlab_server/spec.py	93.75% <80.00%> (-6.25%)	⬇️

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