main/python3: add pythonispython3 subpackage

allows calling python and having it resolve to python3

this is useful because some things hardcode /usr/bin/python /usr/bin/env python, etc (while still wanting python3 explicitly), and it's easier to use this (especially for users) than having to make it themselves, or having to fix/patch third party code. we already were doing something like this in aports (patches or ln -s manually), this just makes it less effort (and doesn't actually break anything)