function annotation raise "ValueError: Function has keyword-only arguments or annotations, use getfullargspec() API which can support them"

[Valérian Rousset]

Python3 introduce (in PEP 3107) the function annotation concept, meaning that one can add information to a function declaration, which can be for example, be use to add types (as in PEP 484).

```
def f(s: str) -> None:
     pass
```
means that a it is a function taking a `str` and returning nothing.

Cliff fail if a function has ones, because it is using `inspect.getargspec` instead of the new `inspect.getfullargspec` in Python3.

I'm attaching a patch to resolve it.