Fails to parse backslash on line by itself

```
% cat parse.py 
class Plotter:
\
    pass
% black parse.py 
error: cannot format parse.py: Cannot parse: 3:4:     pass

Oh no! 💥 💔 💥
1 file failed to reformat.
% black --version
black, 24.2.0 (compiled: yes)
Python (CPython) 3.12.0
% python parse.py
% 
```

Saw this in astral-sh/ruff#10099; the example is minified from the repro case in that issue.