Should python function argument definitions be indented 4 or 8 spaces?

YAPF recently [changed](https://github.com/google/yapf/issues/793) from indenting like

```python
def f(
a: str,
b: str,
) -> None:
pass
```

to:

```python
def f(
a: str,
b: str,
) -> None:
pass
```

I thought the first way was correct, but looking at the style guide more, I'm not actually sure what the correct behavior is. The [indentation section](https://github.com/google/styleguide/blob/gh-pages/pyguide.md#s3.4-indentation) doesn't say anything about function arguments in a def statement. The [type annotation line breaking section](https://github.com/google/styleguide/blob/gh-pages/pyguide.md#3192-line-breaking) does have examples, but those follow Google's internal 2-space indentation style instead of the external 4-space style.