Z3c.breadcrumb

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- Nothing changed yet.

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- Drop support for Python 2.6, 2.7, 3.3, 3.4.

- Add support for Python 3.7, 3.8, 3.9, 3.10, 3.11.

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- Standardize namespace __init__

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- Added support for Python 3.3.

- Moved ``zope.app.testing`` dependency to ``zope.site.testing``.

- Moved ``zope.app.container`` dependency to ``zope.container``.

- Replaced deprecated ``zope.interface.implements`` usage with equivalent
``zope.interface.implementer`` decorator.

- Dropped support for Python 2.4 and 2.5.

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- Added needed but not declated test dependency on ``zope.app.container``.

- Using Python's ``doctest`` module instead of depreacted
``zope.testing.doctest``.

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- Feature: Added ability to register a Null-adapter as a breadcrumb. A
null-breadcrumb will cause the item not to be displayed in the breadcrumbs.