Upgrade six to 1.16.0

[moderation]

While working with the dependencies within Envoy Proxy I tested upgrading the six dependency to 1.12.0. The test failed as protobuf currently uses version 1.10.0 of six. See:
https://github.com/protocolbuffers/protobuf/blob/master/six.BUILD#L3
https://github.com/protocolbuffers/protobuf/blob/master/WORKSPACE#L14-L15

1.10.0 was released in October 2015. The latest version 1.12.0 was released in Dec 2018 and contains several updates and changes - https://github.com/benjaminp/six/blob/master/CHANGES

What is the impact of upgrading the six dependency? Will projects like Envoy Proxy that rely on protobuf break on a six dependency update?

[moderation]

six is now at 1.16.0 - https://github.com/benjaminp/six/releases

1.16.0
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- Pull request #343, issue #341, pull request #349: Port _SixMetaPathImporter to  Python 3.10.

1.15.0
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- Pull request #331: Optimize `six.ensure_str` and `six.ensure_binary`.

1.14.0
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- Issue #288, pull request #289: Add `six.assertNotRegex`.
- Issue #317: `six.moves._dummy_thread` now points to the `_thread` module on  Python 3.9+. Python 3.7 and later requires threading and deprecated the  `_dummy_thread` module.
- Issue #308, pull request #314: Remove support for Python 2.6 and Python 3.2.
- Issue #250, issue #165, pull request #251: `six.wraps` now ignores missing  attributes. This follows the Python 3.2+ standard library behavior.

1.13.0
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- Issue #298, pull request #299: Add `six.moves.dbm_ndbm`.
- Issue #155: Add `six.moves.collections_abc`, which aliases the `collections`  module on Python 2-3.2 and the `collections.abc` on Python 3.3 and greater.
- Pull request #304: Re-add distutils fallback in `setup.py`.
- Pull request #305: On Python 3.7, `with_metaclass` supports classes using PEP  560 features.

Original issue to upgrade to 1.12.0 was closed by #6391