Write a function to get a Python version matrix

[nathanjmcdougall]

Motivation
When adding CI config with usethis ci we usually want a version matrix to test across multiple versions of Python. We need a way to get the relevant versions to add to the matrix.

Summary of desired feature
A function which reads pyproject.toml in the [project.requires-python] section to determine the python version matrix.

Alternatively, it could use the uv lockfile to get this info, or another uv interface.