CI: Check for whitespaces before class
xref #28209, https://github.com/python-sprints/pandas-mentoring/issues/161

Validate that we use
```python
class Foo:
    """
    Doc.
    """
    pass
```
instead of:
```python
class Bar:

    """
    Doc.
    """
    pass
```
(note the blank line between `class  Bar:` and the docstring.