CI cleanup

[blink1073]

• Fix pytest flag in test_sdist
• Test against pypy-3.8 (pypy-3.7 is still used in integration test job).

[codecov-commenter]

Codecov Report

Merging #824 (16474c5) into main (6d84507) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #824      +/-   ##
==========================================
- Coverage   70.20%   70.16%   -0.05%     
==========================================
  Files          63       63              
  Lines        7480     7480              
  Branches     1253     1253              
==========================================
- Hits         5251     5248       -3     
- Misses       1849     1851       +2     
- Partials      380      381       +1     

Impacted Files	Coverage Δ
jupyter_server/services/kernels/kernelmanager.py	78.64% <0.00%> (-0.98%)	⬇️

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