Security: bump lxml (CVE-2026-41066) and pytest (CVE-2025-71176) dependencies

[jvanderaa]

Summary

Two Renovate security-update PRs are open against develop but blocked on the changelog CI check because Renovate doesn't author towncrier fragments. Tracking both bumps here so we can replace the Renovate PRs with branches that include the required changes/*.security fragments.

Updates

• lxml 6.0.2 → 6.1.0 — CVE-2026-41066 / GHSA-vfmq-68hx-4jfw. XXE in iterparse() / ETCompatXMLParser default config (runtime dependency). CVSS 7.5 High. Supersedes Update dependency lxml to v6.1.0 [SECURITY] #400.
• pytest 9.0.2 → 9.0.3 — CVE-2025-71176 / GHSA-6w46-j5rx-g56g. Insecure tmpdir handling under /tmp/pytest-of-{user} enables local DoS / privilege escalation (dev dependency only). CVSS 6.8 Medium. Supersedes Update dependency pytest to v9.0.3 [SECURITY] - autoclosed #396.

Plan

1. New branch per dep from develop, cherry-pick the Renovate lock update, add a changes/<PR#>.security fragment, open a fresh PR.
2. Once merged, close Renovate Update dependency lxml to v6.1.0 [SECURITY] #400 and Update dependency pytest to v9.0.3 [SECURITY] - autoclosed #396.

[jvanderaa]

Closing — the supersede-with-new-branch approach was incorrect. Will revisit once the right pattern is agreed.