[autodoc] please avoid evaluating default function arguments

Currently if Sphinx encounters a function like this:

``` python
def age(self, now=datetime.datetime.utcnow()):
```

It generates output like this:

```
age(now=datetime.datetime(2016, 8, 5, 7, 33, 8, 845335))
```

I think it is confusing and should be avoided.
(Originally reported at [Debian #833820](https://bugs.debian.org/833820).)
