Coverage on CI is failing to report coverage

[krassowski]

1. Since Test more python versions #1358 the coverage gets skipped as some Python versions fail
2. Even before it does not report the coverage correctly, for example at 90133b7 it shows " No data to report."

and there are some errors which do not fail the job:

https://github.com/ipython/ipykernel/actions/runs/13475482660/job/37654246364#step:3:130

Artifact coverage-data.zip successfully finalized. Artifact ID 2635491504
Artifact coverage-data has been successfully uploaded! Final size is 523720 bytes. Artifact ID is 2635491504
Artifact download URL: https://github.com/ipython/ipykernel/actions/runs/13475482660/artifacts/2635491504
The 9 artifact(s) have been successfully merged!
Artifact 'coverage-3556' (ID: 2635490320) deleted
Artifact 'coverage-124[93](https://github.com/ipython/ipykernel/actions/runs/13475482660/job/37654246364#step:3:98)' (ID: 2635485998) deleted
Artifact 'coverage-21348' (ID: 2635486680) deleted
Artifact 'coverage-7093' (ID: 2635486884) deleted
Artifact 'coverage-14416' (ID: 2635491187) deleted
Artifact 'coverage-1062' (ID: 2635487118) deleted
Artifact 'coverage-8930' (ID: 2635486042) deleted
Artifact 'coverage-15[94](https://github.com/ipython/ipykernel/actions/runs/13475482660/job/37654246364#step:3:99)' (ID: 2635487389) deleted
Artifact 'coverage-22413' (ID: 2635489105) deleted
The 9 artifact(s) have been deleted
Run set -eux
+ python -Iim coverage combine
Traceback (most recent call last):
  File "<frozen runpy>", line 1[98](https://github.com/ipython/ipykernel/actions/runs/13475482660/job/37654246364#step:3:103), in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/opt/hostedtoolcache/Python/3.13.2/x64/lib/python3.13/site-packages/coverage/__main__.py", line 10, in <module>
    sys.exit(main())
    ~~~~~~~~^^^^^^^^
SystemExit: 1
>>> 
No data to combine
+ python -Iim coverage html --skip-covered --skip-empty
Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/opt/hostedtoolcache/Python/3.13.2/x64/lib/python3.13/site-packages/coverage/__main__.py", line 10, in <module>
    sys.exit(main())
    ~~~~~~~~^^^^^^^^
SystemExit: 1
>>> 
No data to report.
+ python -Iim coverage report
+ sed 's/^/    /'
Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/opt/hostedtoolcache/Python/3.13.2/x64/lib/python3.13/site-packages/coverage/__main__.py", line 10, in <module>
    sys.exit(main())
    ~~~~~~~~^^^^^^^^
SystemExit: 1
>>> 
+ python -Iim coverage report --fail-under=80
Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/opt/hostedtoolcache/Python/3.13.2/x64/lib/python3.13/site-packages/coverage/__main__.py", line 10, in <module>
    sys.exit(main())
    ~~~~~~~~^^^^^^^^
SystemExit: 1
>>> 
No data to report.

[krassowski]

Once this is back and running we should adjust fail_under threshold.