Remove python-future requirement for Python >= 3.13

[gwuen]

The future package is not compatible with the latest Python 3.13 and apparently there is no prospect of this changing in the near future: PythonCharmers/python-future#640

If I understand the functionality of future correctly, it only overrides the builtins module when using Python 2.x. Since I could not find any other uses of this package in this project, that likely means the dependency can be dropped without any impact for Python 3.13, if not Python >= 3.0.

This might also be the case for DomainConnectDDNS-Python, but I don't want to create two identical issues for now.