The pyqt5 you have is for the version of python that comes with debian not the one you just installed. This is why you don't install from source if you can help it. It makes dealing with dependencies and maintaining the system harder.

My suggestion is that you add debian testing to your list of repos and install pyqt5 from there. It has the updated version. If that doesn't work remove the python version you installed from source and then install the one on testing.

You can use your apt preferences file to only install certain packages from testing repos.