Custom python aggregators create an unneccessary python2 dependency

[angelhof]

Custom python aggregators are not compatible with python3 and require python2 see and therefore create an unneccessary python2 dependency. This should be removed and custom python aggregators should work with Python 3.

@janekb04 Could you take a look?

[nvasilakis]

@angelhof great point, I can take care of this. (@janekb04 is working on the (heavily) optimized C++ versions.)

[angelhof]

When this is done, we should remember to remove the python dependency from the ./scripts/distro_deps.sh script.

[angelhof]

Let's close this after #637 is merged!