Fix Python dependency versions and do github action pinning

**Is your feature request related to a problem? Please describe.**
The goal here is to avoid future issues like the recent LiteLLM CI hack (see https://docs.litellm.ai/blog/security-update-march-2026).

**Describe the solution you'd like**
For Python dependencies -> fix the versions. Update only when required, ideally after a cold down period after releases (if possible).
For Github action -> do action pinning with SHA instead of version.

Better be safe than sorry :)