[CI/Build] Add support for Python 3.12

[mgoin]

This builds locally for me using Python 3.12.4. This was pending on torch==2.4.0 and vllm-flash-attn==2.6.1, and those have landed now.

I have tested installing all CUDA and dev dependencies, along with running a few model tests locally.

FIX #1218
FIX #6877
FIX #6990

[github-actions]

👋 Hi! Thank you for contributing to the vLLM project.
Just a reminder: PRs would not trigger full CI run by default. Instead, it would only run fastcheck CI which consists a small and essential subset of CI tests to quickly catch errors. You can run other CI tests on top of default ones by unblocking the steps in your fast-check build on Buildkite UI.

Once the PR is approved and ready to go, please make sure to run full CI as it is required to merge (or just use auto-merge).

To run full CI, you can do one of these:

• Comment /ready on the PR
• Add ready label to the PR
• Enable auto-merge.

🚀

[simon-mo]

I think this doesn't matter anymore since we shipped py38-abi agnostic version

[mgoin]

@simon-mo This is likely true for installing the built wheel, but before this PR you couldn't build vLLM from source on 3.12 because of PYTHON_SUPPORTED_VERSIONS in the CMakeLists.txt file.