Allow stderr for python3-gi autopkgtests

mesa 24 is now logging

MESA: warning: ZINK: vkCreateInstance failed (VK_ERROR_INCOMPATIBLE_DRIVER)

The mesa developers would like to keep emitting that error in case there is an actual issue with the driver and would prefer not to change that behavior

https://gitlab.freedesktop.org/mesa/mesa/-/issues/10293

This would fix the autopkgtest regression triggered by mesa 24 in Experimental

https://release.debian.org/britney/pseudo-excuses-experimental.html#mesa

cc/ @smcv Does this seem reasonable?