Previously when packages that required the git fetcher were updated, we
would wrongly rely on `nix-prefetch-url`, which would reliable break the
hash.

Instead we need to use `nix-prefetch-git` to determine the proper hash,
when the relevant attributes are present.

All Python packages now have an updateScript. The script calls
`update-python-libraries` and passes it the position of the derivation
expression obtained using `meta.position`. This works fine in case a Nix
expression represents only a single derivation. If there are more in it,
`update-python-libraries` will fail.

This commit introduces two new features:

1. specify with --target whether major, minor or patch updates should be made
2. use --commit to create commits for each of the updates