Revert "Appease new flake8 B028 error:"

This reverts commit 44c2d89.

Author: rwgk

pybind11/setup_helpers.py

@@ -174,9 +174,7 @@ def cxx_std(self) -> int:
     @cxx_std.setter
     def cxx_std(self, level: int) -> None:
         if self._cxx_level:
-            warnings.warn(
-                "You cannot safely change the cxx_level after setting it!", stacklevel=1
-            )
+            warnings.warn("You cannot safely change the cxx_level after setting it!")
 
         # MSVC 2015 Update 3 and later only have 14 (and later 17) modes, so
         # force a valid flag here.