`WPS226` false positive on fstring parts

This code raises a violation:

```python
x = f'single {1}'
y = f'single {1}'
```

When `--max-string-usage=1` is provided.

Output:

```
  1:5      WPS226 Found string literal over-use: single  > 1
  x = f'single {1}'
      ^
```

This does not seem right, because strings that are part of `f` strings should not be counted by `WPS226`

PR would be rather easy :)