Upstream has worked on porting mnemosyne to Python3 and PyQt5. They have published a link to a beta for the upcoming release 2.4 of mnemosyne on their mailinglist:
https://groups.google.com/forum/#!topic/mnemosyne-proj-users/Et6uWBbFRjU This version of mnemosyne should be compatible with the PyQt5 in Debian testing. It would be nice if we could package this before the release of stretch. Cheers, Felix