Add `requires-python` to `uv init` (#5322)
## Summary
Prefers, in order:
- The major-minor version of an interpreter discovered via `--python`.
- The `requires-python` from the workspace.
- The major-minor version of the default interpreter.
If the `--python` request is a version or a version range, we use that
without fetching an interpreter.
Closes https://github.com/astral-sh/uv/issues/5299.
