Description

• Add cache: 'pip' and cache-dependency-path to all actions/setup-python@v6 steps in .github/workflows/test.yml to speed up pip install via cache reuse while retaining the explicit pip install step (so dependencies still update to the latest or specified version per requirements).
• Change runs-on for the experimental test-python-315-experimental job from ubuntu-latest to ubuntu-26.04, since 3.15.0-beta.4 Linux packages are now available at ubuntu-26.04 (see actions/python-versions release 3.15.0-beta.4-29712574371). This unblocks the previously-failing experimental jobs.

Background

• actions/python-versions PR #402 merged on 2026-07-20 added 3.15.0-beta.4 to the manifest, with linux-22.04/24.04/26.04 packages.
• actions/runner-images shipped ubuntu-26.04 and ubuntu-26.04-arm64 GA images starting 2026-07-14 (release ubuntu26/20260714.74).
• actions/setup-python v6/v7 supports cache: 'pip' only — there is no built-in run-pip input. The cache stores ~/.cache/pip downloads; pip install still runs (correctly installing/updating per requirements*.txt content) but with no re-download cost on cache hit.

Type of change

• ci: Changes to our CI configuration files and scripts
• chore: Changes to the build process or auxiliary tools and libraries

Breaking Change

• Yes
• No