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.
Thanks for your reply.
Below is the advice from Debian (paragraph lifted from "Don't make a FrankenDebian"
https://wiki.debian.org/DontBreakDebian):
Debian Stable should not be combined with other releases. If you're trying to install software that isn't available in the current Debian Stable release, it's not a good idea to add repositories for other Debian releases. The problems might not happen right away, but the next time you install updates.
The reason things can break is because the software packaged for one Debian release is built to be compatible with the rest of the software for that release. For example, installing packages from buster on a stretch system could also install newer versions of core libraries including libc6. This results in a system that is not testing or stable but a broken mix of the two.
Repositories that can create a FrankenDebian if used with Debian Stable:
Debian testing release (currently buster)
Debian unstable release (also known as sid)
Ubuntu, Mint or other derivative repositories are not compatible with Debian!
Ubuntu PPAs
Conclusion: I will just not use Electrum 3.3.2 on my Debian system.