`noqa` comments parsing logic

Currently these are all valid `noqa` comments:

```py
# noqa: A100,B101
# noqa: A100 B101
# noqa: A100, B101
# noqa: A100 , B101
```

```py
# noqa: A100B101
# noqa: A100,,B101
```

The formats in the second group should be invalid.
