Jenkins complains for python 2.6
DEPRECATION: Python 2.6 is no longer supported by the Python core team
ERROR: InvocationError: '/tmp/protobuf/protobuf/python/.tox/py26-cpp/bin/python setup.py -q build_py'
https://grpc-testing.appspot.com/job/protobuf_pull_request/1502/testReport/junit/(root)/python_cpp/python_cpp/