Control: reassign -1 src:python3-defaults,src:dh-python
I'm not sure, if the pre-dependency is the correct solution (or the only solution). To run py3clean/py3compile, you need any python3.X package unpacked. So a possible solution could be to make py3clean/py3compile shell scripts, which try to find a valid python3 interpreter first. Using python3, if it's not a dangling link, or else any python3.X else.
