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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.