pycodestyle: W604 / backticks are deprecated, use 'repr()'

Hi!
This is not the expected behavior.

W604.py:
```python
var = 'test'
print(f'var: `{var}`')
```

```bash
> prospector W604.py
W604.py
  Line: 2
    pycodestyle: W604 / backticks are deprecated, use 'repr()' (col 14)
```