Note that we did the move to PyQt5 because Ubuntu was getting rid of support of PyQt4 and QtWebkit :-) Also, as far as I know, Ubuntu 17.04 does have Mnemosyne packages.
Rather than trying to downgrade everything to 2.3.6, I strongly suggest upgrading to 2.4.1. Personally, I run 2.3.6 on a headless Ubuntu server 16.04.2. There is no dependency on Qt if you install Mnemosyne from source.
Note that we did the move to PyQt5 because Ubuntu was getting rid of support of PyQt4 and QtWebkit :-) Also, as far as I know, Ubuntu 17.04 does have Mnemosyne packages.
Rather than trying to downgrade everything to 2.3.6, I strongly suggest upgrading to 2.4.1. Personally, I run 2.3.6 on a headless Ubuntu server 16.04.2. There is no dependency on Qt if you install Mnemosyne from source.