issue with python3 dictionary unpacking

[PEP 448](https://www.python.org/dev/peps/pep-0448/) dictionary spreading creates invalid syntax

```
x = {}
merged = {**x}
```
becomes
```
x = {}
merged = {: x}
```

using git master version of plugin: 

`git+https://github.com/prettier/plugin-python.git#036c8a66c166ed710b2a0d280e4c85015a1a34c5` (`"version": "0.0.0-development"`)