Appease new flake8 B028 error

[rwgk]

Description

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!")
            ^

Suggested changelog entry:

[rwgk]

@henryiii What do you think about this simple tweak to make flake8 happy?

[rwgk]

Thanks Aaron!

[henryiii]

I think we need to move to Ruff, so these updates won't happen behind the scenes. :) Yes, I think this is fine. Though this is almost always =2, and I think that's true there - the problem is in the user using the cxx_std setter (=2), not inside the setter itself (=1). But this does just formalize the current (wrong) behavior, which is why the check exists.

[rwgk]

I think we need to move to Ruff,

What I saw looked exciting to me, but I see there are a lot of GHA failures. Do we need to pull together to migrate to Ruff? (The sooner the better.)

the problem is in the user using the cxx_std setter (=2)

Thanks! I'll send another PR to change the value to 2.