When we create a source tarball via `python setup.py sdist` on Python 2, the generated protocol buffer code includes relative imports.
Because of that, a tarball created on Python 2 interpreter cannot be used on Python 3.
AFAIK, we release only one source tarball to PyPi, so if possible we should make source distribution of Beam compatible both with Python 2 and Python 3.
When we create a source tarball on Python 3, we call futurize on generated _.pb2.py files. Looks like this does not happen on Python 2, which we can fix in this issue.