Keras Add daily CPU tests which also test with the latest Python version

Today, all the tests are run with the lowest supported version of Python (currently 3.10).

We want to also test with the latest version of Python (3.13). However, probably not for every PR.

We should therefore add daily tests, which can be CPU only, that test with 3.13. They can be part of the nightly workflow: https://github.com/keras-team/keras/blob/master/.github/workflows/nightly.yml

This is how tests are run nightly in keras-hub: https://github.com/keras-team/keras-hub/blob/ecef6d1cd43e34729d6c759d34a26d39560a0a70/.github/workflows/nightly.yml#L12-L13