[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