Installing the binary plugins with "hp-plugin -i" fails because the installer for the binary stuff from HP runs "python" from the path, which is python 2. But hpmudext.so is built against python3. So either please re-build hplip against python2 OR put a "python" that links to python3 in the PATH before /usr/bin when running the installer.
Sorry for the noise. *** This bug has been marked as a duplicate of bug 1196237 ***