Support `python-version-file` argument for `setup-python`

[evansd]

This would allow us to avoid repeatedly hardcoding the Python version throughout the workflow files and give us a single place to update:
https://github.com/actions/setup-python/blob/main/docs/advanced-usage.md#using-the-python-version-file-input

I imagine using it like:

steps:
  - uses: "opensafely-core/setup-action@v1"
    with:
      python-version-file: ".python-version"
      cache-dependency-path: "requirements.*.txt"
      install-just: true

[evansd]

Just to note that uv supports this out-of-the-box so we'd get it for free in:

• Support installing uv #17

[alarthast]

Closed in #21