flake8 should only consider a noqa comment if it is actually a comment

minimal reproduction:

```python
def f():
    x = '# noqa'
```

I expect this to report the same as if that were an unrelated string:

```python
def f():
    x = '......'
```

```console
$ flake8 unrelated.py
unrelated.py:2:5: F841 local variable 'x' is assigned to but never used
```