Log Message:
devel/{py-,}protobuf: Update to 3.4.1

Notable changes:

* C++
  - Proto3 messages are now able to preserve unknown fields.
  - Packable proto3 fields are now packed by default in serialization.

* Python
  - SerializeToString API is changed to SerializeToString(self, **kwargs).
  - Added sort_keys parameter in json format to make the output
    deterministic.