Switch to `python-constraints2`

Since [`python-constraint2`](https://pypi.org/project/python-constraint2) is not in beta anymore, it might be that the problems in #293 are not an issue anymore. Also note that binaries are provided for 3.14 as well since [v2.5.0](https://github.com/python-constraint/python-constraint/releases/tag/2.5.0).