Python bindings should be in a separate output

[FRidh]

Issue description

Many libraries come with Python bindings. These are typically built with the default Python interpreter. As we're switching to Python 3, this is causing some issues, since many cannot be built with Python 3.

What we need to do is:

• put Python bindings in a separate output, py.
• refer to these bindings from python-packages.nix.

Then, when a package requires those Python bindings, you would include pythonXXPackages.thebindings, and preferably with python.withPackages.

See for example #20251.

[vcunat]

In that example the bindings seem hard-coded to the python2 passed to the original build. In general I don't see how you intend them to react to pythonXXPackages version.

[FRidh]

in python-packages.nix the python2 that is passed could be a python3, but because the packages are not compatible with python3 an exception is raised. I think we should use the pythonPackages.package versions if the bindings are required, since that will prevent mixing python2 and python3 packages as is happening I think with the python3 branch (again).

Note I made a small error in the case of libxml2, this example is perhaps better
dbdda72

[FRidh]

This is now done with several packages as is clear from looking at python-packages.nix. So far, I don't think there are any issues with this approach.