Add matching for f-string interpolation brackets

I find it difficult to see at a glance where f-string interpolation is happening. 

Where:

```python
print(f'hello {name}')
```

would add syntax matching for the `{` and `}` characters.

Also, even though general syntax highlighting for the interpolated `name` expression seems to be beyond vim's capabilities at the moment, it might be nice to match "everything inside the brackets" too. 