Fix unable detect qt5 by using high version scons to build

[OmegaZhou]

I use Python3 and Scons-v4.3 to build the mitsuba, but was unable to detect the qt5 when building. I try to get the exception from qt5.py, and I find that SCons.Warnings do not have the attribute of Warning When I search the document of Scons-v4.0.1, I see the Warning is inherited from Errors.UserError and has subclass WarningOnByDefault However, from version v4.1.0, I can't find the Warning in the document, instead I find class SConsWarning is inherited from Errors.UserError and has subclass WarningOnByDefault So I change the qt5.py so that it can run in high version Scons and keep it compatible with the low versions.