As said in Debian's python policy I'm aware of it. Let me clarify this point here. one of the build dependencies of python-caffe-* is python*-protobuf, and python3-protobuf is not possible for current protobuf version in Debian. In a word, build dependencies not satisfied for python3-caffe-*.
i am in the process of packaging a python3 compatible version of protobuf. i don't think it's API compatible with the official google version, but it is close. caffe would require patching, but perhaps not too much.
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=823478 it's probably not feasible, but i just thought i'd mention it. jonathon