A better way to specify a Python prerelease version

[dataflake]

Right now the value of FUTURE_PYTHON_VERSION in shared/packages.py is set as a range, currently 3.13.0-alpha - 3.13.0. That's a bit clunky because that version string is inserted in many places.

The documentation for the setup-python GH action mentions a cleaner way that uses the flag allow-prereleases: true, so you could define FUTURE_PYTHON_VERSION as a "clean" 3.13 and write it all like this:

- uses: actions/setup-python@v4
  with:
    python-version: "3.13"
    allow-prereleases: true

See https://github.com/actions/setup-python/blob/main/docs/advanced-usage.md#allow-pre-releases.

[icemac]

This looks interesting and I like it more than the range.