Convert protoc-gen-nanopb to python 3 am: cd9c9aa9f1 am: 495c9a97b5 am: e39787e039

Original change: https://android-review.googlesource.com/c/platform/external/nanopb-c/+/2208439

Change-Id: Iadc4e5106e64aece4ae7c93626747e81b73c2be1
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>