Update to use actions/setup-python@v6

[slepkin]

Whenever this action runs, we see the following notification:

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/setup-python@v5.
For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

I think we can fix this just by using actions/setup-python@v5. https://github.com/actions/setup-python/releases/tag/v6.0.0