python3-pb2nano is a minimal protobuf2 reader and writer.
Currently it doesn't have a protocol parser, so you have to specify your protocols with python objects. Furthermore it only supports unsigned varints, string, bytes and top-level enums and messages.
It was created for python3-ipfs-api to parse very simple protobuf2 messages and is meant as a replacement until Google adds proper Python3 support for Protocol Buffers.
Sorry, but no documentation currently available.