buildPythonPackage: use fetchFromGitHub if tests are missing on Pypi

[pbsds]

[figsoda]

This is going to be hard. nix-init doesn't yet try to build the package, and even if it does, it would be hard to determine that the tests failing are due to missing assets on the pypi release. Determining the fetcher also happens before everything else, so the user would have to go through all the prompts again if this feature does get implemented