Strange operator alignment with powers of string literals

yapf (tested with 2c13f6f) turns the following:

```python
b"" ** b""
```

into the following:

```python
b"" **b""
```

pycodestyle calls this "E225 missing whitespace around operator" but actually it's fine with either `x**y` or `x ** y` (and the former is what yapf normally produces). The oddity here is that there's whitespace on one side but not the other.