Context

The toolchain still builds and tests Python 3.9 and PyPy 3.9/3.10, which are end of life. It has no coverage for the upcoming Python 3.15 release, and several patch versions and dependencies have fallen behind the latest available releases.

Definition of done

• Python 3.9, PyPy 3.9, and PyPy 3.10 are removed from the toolchain.
• Python 3.15 RC is added to the toolchain.
• All Python and PyPy patch versions are bumped to the latest release with binaries available.
• Toolchain dependencies are bumped to their latest versions.
• Implementation notes: https://gist.github.com/blink1073/539f11337b9f61561a44098f2e736211

Pitfalls

• Free-threaded build and finalize steps must cover the new Python 3.15 build on Linux, macOS, and Windows.
• Sigstore verification needs the correct release-manager identity for Python 3.15.
• A full Evergreen build across all platforms is required before merging; syntax checks alone don't confirm the toolchain builds.