Z3c.relationfield

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- Nothing changed yet.

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* Replace ``pkg_resources`` namespace with PEP 420 native namespace.

* Drop ``xml`` extra and all the code which was enabled by it.
(``z3c.schema2xml`` is unmaintained for years now.)

* Drop support for Python 3.8.

==================

- Revert: Consider RelationValue without source as broken.
[ksuess]

- Add support for Python 3.12, 3.13.

- Drop support for Python 3.7.

- Fix tests for Python 3.13.
[petschki]

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- Consider RelationValue without source as broken.
[ksuess]

================

Breaking changes:

- Drop support for Python 2.7, 3.5, 3.6.

New features:

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

==================

New features:

- Provide IRelationBrokenEvent to be able to distinguish the event when
subscribing to IObjectModifiedEvent
[vangheem]