[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

- github.com/coreos/pkg: [97fdf19 → 399ea9e](https://github.com/coreos/pkg/compare/97fdf19...399ea9e)
- github.com/docker/docker: [be7ac8b → aa6a989](https://github.com/docker/docker/compare/be7ac8b...aa6a989)
- github.com/docker/go-connections: [v0.3.0 → v0.4.0](https://github.com/docker/go-connections/compare/v0.3.0...v0.4.0)

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