fix hellogrpc/py examples by adding dependency

[erain]

• All current Travis CI would fail because hellogrpc/py e2e test would
  fail. This PR fix this test, so that Travis CI works again.

• hellogprc/py server would produce a py_image which is failed to
  start because of missing a dependency:

Traceback (most recent call last):
  File "/app/hello_grpc/py/server/server.binary.runfiles/__main__/hello_grpc/py/server/server.py", line 1, in <module>
    import grpc
  File "/app/hello_grpc/py/server/server.binary.runfiles/pypi__grpcio_1_6_0/grpc/__init__.py", line 22, in <module>
    from grpc._cython import cygrpc as _cygrpc
  File "src/python/grpcio/grpc/_cython/cygrpc.pyx", line 17, in init grpc._cython.cygrpc (src/python/grpcio/grpc/_cython/cygrpc.c:29041)

• Adding setuptools in requirements fixes the problem.

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @erain. Thanks for your PR.

I'm waiting for a bazelbuild member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[erain]

/ok-to-test

[erain]

I don't know why my /ok-to-test doesn't work (I am a member of bazelbuild).

Merge this PR since it will fix Travis CI and unblock other PRs to merge.