SDK source tarball is different when created on Python 2 and Python 3

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[1]. Looks like this does not happen on Python 2, which we can fix in this issue.

[1] [https://github.com/apache/beam/blob/d6dc3602c808ce1961f46098deb898e5c7480ad4/sdks/python/gen_protos.py#L122

cc altay, charleschen70@yahoo.com, RobbeSneyders