Zope-annotation

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- Nothing changed yet.

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- Make ``AttributeAnnotations`` have a ``__parent__``. The
``__parent__`` is the object that it stores ``__annotations__`` on.
This is a convenience for upwards traversal as used by things like
``zope.keyreference``. See
https://github.com/zopefoundation/zope.annotation/issues/11

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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.