Fix for #3623

Make sure that the imported qt backend is consistent with selected backend. Otherwise the combination of the ETS env variable and a selected backend can result in matplotlib importing PyQt4 and using it inside the qt5 backend and vice versa resulting in bugs such as #3623
Only if the env param tries to use a different backend from the one the user has selected. 

Currently if the ENV is 'pyqt' (i.e. a qt4 backend) but the user has done ```matplotlib.use('Qt5Agg') then you get PyQT4 imported into the matplotlib qt5 backend which makes no sense and is just wrong. This means that if you have that variable defined (as I have to control what backend mayavi uses pyside or pyqt4 ) I can't run the qt5 backend but get a broken mixture of the matplotlib qt5 code and the pyqt4. 

It makes no sense to use the env variable to determine what major Qt version should be imported since that is selected by the choice of backend.