Zope.annotation

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- Drop support for Python 2.6.

- Claim support for Python 3.5 and 3.6.

- Reach 100% test coverage.

- ``AttributeAnnotations`` is now always a
``collections.MutableMapping``. Previously on Python 2 it was a
``UserDict.DictMixin``.

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- Convert doctests to Sphinx documentation. Doctest snippets are still
tested via ``tox -e docs``.

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- LP 98462: add additional "iterable mapping" methods to ``IAnnotations``.

- LP 878265:

- Make ``persistent`` (used only for doctests) a soft dependency,
installable via the ``zope.annotation[btree]`` extra.

- Make ``BTrees`` (used for attribute storage) a soft dependency,
installable via the ``zope.annotation[btree]`` extra. Fall back to
using ``dict`` for attribute storage if ``BTrees`` is not importable.

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- Add support for Python 3.4.

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- Don't make AttributeAnnotations available as a view.

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- Add ``__bool__`` method to ``IAnnotations`` API for Python 3 compatibility.