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 go get github.com/golang/protobuf/protoc-gen-go
Then, regenerate the libraries (must be done outside chroot):
cd protos protoc --python_out=../venv/skylab_inventory/protos/ *.proto protoc --go_out=../go/src/chromiumos/infra/skylab/inventory/protos/ *.proto