Add missing PY2AND3 srcs_versions attributes to Python Bazel build targets

[davidzchen]

This is to fix the build break in tensorflow/tensorflow#1289

+cc @martinwicke @podsvirov

[grpc-kokoro]

Thanks for your pull request. The automated tests will run as soon as one of the admins verifies this change is ok to test.

[xfxyjwf]

@haberman to review the change. Do we currently have test coverage for bazel on Jenkins?

[davidzchen]

Ping

[podsvirov]

Hello @davidzchen!
I love C ++ and well versed in CMake.
I contributed to the project in terms of improvement protobuf CMake project.
How can I help in this case?
I read about a variable srcs_version = "PY2AND3", but do not know where it can be used in C ++ code? Teach me :-)

[davidzchen]

@podsvirov, thanks for your enthusiasm. This particular PR, and the srcs_version = "PY2AND3" attribute, is specifically for the Bazel build for protobuf. I don't think that the CMake build for protobuf has this issue.

[xfxyjwf]

ping @haberman

[davidzchen]

Can someone take a look at this? This is blocking tensorflow/tensorflow#1289.

[martinwicke]

Thanks!