Unable to parse varargs with empty name (Keyword-Only Arguments)

``` python
def f(a: int, *, b: int = ...) -> None: ...
```

gives with pytd

```
Error while parsing pyi:
  File "asd3.pyi", line 1
    def f(a: int, *, b: int = ...) -> None: ...
                   ^
SyntaxError: Unexpected 'COMMA'
```

But this notation was introduced in PEP 3102, and IMO should be valid in stub files (it is already used in typeshed).
