Shortcut in Python 2.7 syntax for "don't care about arguments"

Proposal: in addition to the `# type: (t1, t2) -> t3` syntax already added to the PEP, also let us write `# type: (...) -> t` (with a literal ellipsis). This saves having to count the args. Example:

```
def many_args(x, y, size, tuning, theta):
    # type: (...) -> List[float]
    <clever math code>
```
