BUG: fixes bug when using sep=None and comment keyword for read_csv
- [x] closes #31396 
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry - added

When adding `dict(sep=None)` as parameter to `test_line_comment` I found that my current fix does not work if the first line is a comment.

~~Overall I think the flow of comment removal and separating columns is a bit inconsistent. During searching for the bug I found that `PythonParser._check_comments` is sometimes called before separating the line (e.g. when sniffing the separator) and sometimes afterwards. In my opinion it would make more sense  to remove comments first in all cases.~~

The next commit fixes the issue with comments on the first line. The Travis CI build failed with the following message:
```
UnavailableInvalidChannel: The channel is not accessible or is invalid.

  channel name: c3i_test

  channel url: https://conda.anaconda.org/c3i_test

  error code: 404
```

Is there a way to rerun it? @s-scherrer : This is actually an unrelated error that we need to resolve on `master`.

cc @pandas-dev/pandas-core thanks @s-scherrer 