Wrap the concurrent futures in an asyncio future

[blink1073]

Follow up to #988

[codecov-commenter]

Codecov Report

Base: 72.42% // Head: 72.38% // Decreases project coverage by -0.04% ⚠️

Coverage data is based on head (c054752) compared to base (017c52e).
Patch coverage: 60.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1001      +/-   ##
==========================================
- Coverage   72.42%   72.38%   -0.05%     
==========================================
  Files          64       64              
  Lines        8243     8249       +6     
  Branches     1378     1375       -3     
==========================================
+ Hits         5970     5971       +1     
- Misses       1855     1857       +2     
- Partials      418      421       +3     

Impacted Files	Coverage Δ
jupyter_server/services/kernels/handlers.py	56.96% <60.00%> (-0.72%)	⬇️
jupyter_server/services/kernels/kernelmanager.py	77.99% <0.00%> (-0.65%)	⬇️
jupyter_server/serverapp.py	65.49% <0.00%> (-0.10%)	⬇️
jupyter_server/services/contents/filemanager.py	70.84% <0.00%> (+0.78%)	⬆️

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