[Python 2's EOL on January 1, 2020](https://www.python.org/dev/peps/pep-0373/#update).

Coinciding with this change, new releases of
[TensorFlow's Docker images](https://hub.docker.com/r/tensorflow/tensorflow/)
provide Python 3 exclusively. Because all images now use Python 3, Docker tags
containing `-py3` will no longer be provided and existing `-py3` tags like
`latest-py3` will not be updated.

## Major Features and Improvements

# Linux builds. Instructions are available in the official documentation:
# https://www.tensorflow.org/install/source#install_clang_recommended_linux_only
# Another good option is to use our Docker containers to build and test TF:
# https://github.com/tensorflow/tensorflow/tree/master/tensorflow/tools/tf_sig_build_dockerfiles.
build:unsupported_cpu_linux --config=avx_linux