Cache Python deps in CI

In `.github/workflows/ci.yml`, add pip cache to the Python job to speed up the byte-compile step.

Use `actions/setup-python@v5` with the `cache` option:
```yaml
- name: Set up Python
  uses: actions/setup-python@v5
  with:
    python-version: '3.10'
    cache: 'pip'
    cache-dependency-path: nexum_ai/requirements.txt
```