Drop future dependency for Python 3

[wshanks]

future is only used by uncertainties to provide builtins imports in Python 2, so it is not needed when using uncertainties in Python 3. The main motivation for dropping future now is that it is unmaintained and has unpatched security vulnerabillties (see PythonCharmers/python-future#610 for example). The vulnerabilities do not affect uncertainties but they add a hurdle to some users using uncertainties as they may not want to have known unpatched security vulnerabilities in their environments.

Personally, I think it would be fine to drop Python 2 support entirely but this PR makes the minimal change of just not listing future as a dependency when installing uncertainties in Python 3.