When you're installing a wheel, python doesn't provide 'built time' dependencies (because in the python world, wheels are ready made binaries, while for nix they need to have their LD paths patched).
You need to add final.{pkg providing the .so} to the buildInputs, and something like ''addAutoPatchelfSearchPath "${final.pkg}/${final.python.sitePackages}/${final.pkg.pname}/lib"'' to the preFixup phase.