Reason of memory leak in disabled autoloading of images. It's a bug that will be fixed in next QT version. Removing this line solves the problem for example above:
self.settings().setAttribute(QWebSettings.AutoLoadImages, False)
Second possible reason which can lead to leaks is "Memory leak in GStreamer". It's in process.
Update:
I see people still looking for a solution. I've recently noticed bug with AutoLoadImages=False was not fixed in version Qt 5.2.1, nor in Qt 5.3 RC. New discussion about it has been opened. You can vote for this issue in bugtracker to increase the chances for fix in Qt 5.3.0