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
This script determines whether there is an update available for the
package and if so, updates the version and sha256 attributes.
This only works for packages that have their own expression, and `pname`
and `sha256` attribute.
