Ec2-metadata

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* Add ``ipv6s``, ``subnet_ipv6_cidr_blocks``, and ``vpc_ipv6_cidr_blocks``
attributes to ``NetworkInterface``.

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* Add ``instance_action`` and ``kernel_id`` attributes.

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* Add ``iam_info``, ``instance_profile_arn`` and ``instance_profile_id``
attributes.
* Refactor handling non-200 responses to be more strict for attributes where
404's are allowed.

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* Fix rendering of docs on PyPI.

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* All methods can now raise ``requests.exceptions.HTTPError`` if the metadata
API returns a bad response, rather than failing during parsing or silently
returning data from non-200 responses.
* ``EC2Metadata`` can now be passed a ``requests.Session`` object for
customization of the way requests are made.

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* Make ``public_*`` properties return ``None`` for instances that aren't
public.