CVE-2025-0938: python3, backport upstream fix (no version bump)

[jasonodoom]

⚠️ EXPERIMENTAL source-backport — NOT a version bump

This PR authors the upstream fix as an in-tree patch against the current nixpkgs version (no version bump). An agent generated it; a cross-family reviewer verified it transcribes the upstream commit, and a cert proved the bundled reproducer goes red→green. A human maintainer must still confirm sufficiency before merging:

• This applies the COMPLETE upstream fix, not one of several required commits.
• The backported hunk matches the upstream fix commit(s):
  • python/cpython@d89a5f6
• Reviewed the cert evidence (reproducer red→green) for CVE-2025-0938.

Opened as a draft deliberately — mark Ready-for-review only after the boxes above are checked.

---

Evidence: CVE-2025-0938 (human-review-required) trace bundle 7e4ccb8bb1b71050

• Rekor entry: https://rekor.sigstore.dev/api/v1/log/entries?logIndex=2721458001
• Workflow identity: https://github.com/Ad-Astra-Computing/trace/.github/workflows/steward.yml@refs/heads/main
• OIDC issuer: https://token.actions.githubusercontent.com
• nixpkgs rev: 63ab5cd1c5dde00330fd270a91d13974a87f7bdf

Verify locally

gh run download <run-id> --name steward-smoke-<bundle> --dir ./artifact
trace bundle verify ./artifact  # bundle_id=7e4ccb8bb1b71050

Generated by trace.

[jasonodoom]

Comparison against the upstream fix

Transcription lane (this PR's payload): the added and removed code lines of the embedded patch are identical to the upstream fix commit, normalized for whitespace. The cross-family Reviewer attested this; the check here was recomputed independently against the bundle's upstream-fix.patch.

Blind-authoring lane (separate measurement, not this payload): when the same CVE was given to claude-opus-4-8 with only the vulnerability description and the pre-fix source, never the upstream patch, the graded result against the human fix was: WRONG (tweaked the helper, never changed the caller). Report: trace data/reports/authoring-accuracy-vs-human-2026-09-04.json.

Across the nine review PRs opened 2026-09-05 the blind lane scored 1 exact, 2 equivalent, 3 partial, 3 wrong. The pipeline's Reviewer plus cert gate is what stands between the blind lane's error rate and a merge candidate.