PyEthereum is deprecated, to be replaced by py-evm at https://github.com/ethereum/py-evm/ . This move comes with the blessing of the PyEthereum dev team, which is currently porting their ongoing work over to py-evm. It probably makes sense to include py-evm in this onboarding process. Depending on the length of the process to get included, it might even make sense to skip over PyEthereum altogether.