Post-process python generated files to fix relative import bug

[lyoshenka]

lbrynet cannot directly import the generated python code. I think the problem is that protobuf generates files with absolute import paths, and py3 needs them to be relative so they can be imported.

protobuf issue: protocolbuffers/protobuf#1491

our hacky solution: https://github.com/lbryio/lbry/blob/master/lbrynet/schema/Makefile

This is only necessary if we want to use types repo as a module. An alternative is to copy-paste the proto files into lbrynet repo, and then generate the protobufs there.