Inventory schema is stored as protobuf definitions in the proto/ directory. When you update these protobuf definitions, you must also update the generated python libraries for them.
You need the protoc compiler for this. On gLinux:
$ sudo apt-get install protobuf-compiler
Then, regenerated the libraries:
$ cd protos $ protoc --python_out=../venv/skylab_inventory/protos/ *.proto