fix(ci): Bump Python to 3.12 in E2Es

[andreyvelich]

We should update Python to 3.12 since PyTorch 2.10 uses it.
/cc @kubeflow/kubeflow-trainer-team @akshaychitneni @robert-bell

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In response to this:

We should update Python to 3.12 since PyTorch 2.10 uses it.
/cc @kubeflow/kubeflow-trainer-team @akshaychitneni @robert-bell

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[Copilot]

Pull request overview

Updates the repository’s CI and contributor guidance to run tests/E2Es on Python 3.12, aligning the workflows with the desired Python runtime for the project’s notebook-based E2E coverage.

Changes:

• Bump Python version to 3.12 in the Python unit/integration CI workflow.
• Bump Python version to 3.12 in CPU and GPU E2E GitHub Actions workflows.
• Update CONTRIBUTING requirements to state Python 3.12+.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
CONTRIBUTING.md	Updates documented local dev requirements to Python 3.12+.
.github/workflows/test-python.yaml	Runs Python unit/integration CI on Python 3.12.
.github/workflows/test-e2e.yaml	Runs E2E workflow setup with Python 3.12.
.github/workflows/test-e2e-gpu.yaml	Runs GPU E2E workflow setup with Python 3.12.

[robert-bell]

Hey @andreyvelich I dug into the failing local-container-mnist.ipynb notebook e2e test on #3227 and it looks like python in the pytorch 2.10 image is now enforcing PEP 668, so pip install fails if we're not running in a venv, which is causing custom train jobs created by the sdk to fail. I think that's the root cause of that test failing, though I don't understand why it didn't fail in the 2.10 upgrade PR.

I think we should be safe to get the sdk to just disable the PEP 668 check if we're running running in a container. I'll open a PR.

[andreyvelich]

Yes, let's update it please.
cc @Fiona-Waters @kubeflow/kubeflow-sdk-team

[robert-bell]

I've opened kubeflow/sdk#384 and I've checked that the failing local-container-mnist.ipynb test locally and it's passing.