ci: drop redundant Test PyPI upload step from release workflow

[blink1073]

References

Description

Implements item 1 of the CI/release workflow hardening checklist (based on calysto/metakernel#466): drops the "Upload package to Test PyPI" step from release.yml.

Items 2 and 3 of that checklist are already covered in this repo:

• Static package checks: validate-pyproject already runs via pre-commit in the static job, and twine check --strict already runs as part of hynek/build-and-inspect-python-package (used by the build job on every push/PR).
• Codecov fork-PR fix: already merged in b2c4686 ("ci: allow codecov uploads on fork PRs targeting Calysto/octave_kernel").

Changes

• Remove the "Upload package to Test PyPI" step from release.yml. It doesn't catch anything the existing static build/package checks don't already catch, and it's one more external service in the critical path of every release.

Backwards-incompatible changes

None

Testing

• just pre-commit
• just typing

AI usage

• Some or all of the content of this PR was generated by AI.
• The human author has carefully reviewed this PR and run this code.
• AI tools and models used: Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.93%. Comparing base (cd925fa) to head (ff77784).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #355   +/-   ##
=======================================
  Coverage   92.93%   92.93%           
=======================================
  Files           6        6           
  Lines         439      439           
=======================================
  Hits          408      408           
  Misses         31       31           

Flag	Coverage Δ
linux	92.71% <ø> (ø)
windows	92.93% <ø> (ø)

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