pythonPackages.magic-wormhole: fix build

- fixes build
- use buildPythonPackage and pass in individual packages

As mentioned in https://github.com/NixOS/nixpkgs/pull/20722 this
packages is both an application and a library.

If a package is both then we use buildPythonPackage and put it in
python-packages.nix. That way we can guarantee we use the correct
version of dependencies when using it as a library. Unfortunately, it
does mean the name of the package is prefixed with `pythonX.X-`.