align '--' in Python docstring arg list

Tweak to recently added function python-args-to-docstring.

Now it will align the '--' to look like this:

```
def my_func(foo, barbar=0, bazbazbaz = False)
    """

    Keyword Arguments:
    foo       --
    barbar    -- (default 0)
    bazbazbaz -- (default False)
    """
```