skip_file ignored when on the first docstring line

This does not occur on version `5.9.3`.

Since version `5.10.0`, the following `skip_file` directive is ignored:

```python
"""isort:skip_file"""
```

This is also ignored:

```python
"""isort:skip_file

some other comment.
"""
```

It needs to be on a new line:

```python
"""
isort:skip_file
"""
```