Appease new flake8 B028 error: (#4513)

```
flake8...................................................................Failed
- hook id: flake8
- exit code: 1

pybind11/setup_helpers.py:177:13: B028 No explicit stacklevel keyword argument found. The warn method from the warnings module uses a stacklevel of 1 by default. This will only show a stack trace for the line on which the warn method is called. It is therefore recommended to use a stacklevel of 2 or greater to provide more information to the user.
            warnings.warn("You cannot safely change the cxx_level after setting it!")
            ^
```