Collective.catalogcleanup

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- Nothing changed yet.

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- No longer accept non-unique inherited UIDs.
This was needed for Discussion Items, but not anymore.
[maurits]

- Fixed setting new uuids for Dexterity content and Discussion Items.
This part of version 1.x seems to only have worked for Archetypes.
[maurits]

- Removed support for checking the ``reference_catalog`` and ``uid_catalog``.
These are Archetypes-only.
[maurits]

- Drop support for Python 2, Plone 5.1 and earlier.
Only Plone 5.2 and 6 are supported, on Python 3.
[maurits]

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- Do not complain about paths being different when the brain path is relative.
Happens in old uid_catalog and reference_catalog.
[maurits]

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- Again fixed error sorting UIDs when one of them is None. [maurits]

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- Report and remove brains that have a wrong path.
These are brains where an object is found at the given path,
but the actual path of this object is different.
This can be caused by acquisition:
/Plone/folder/folder has gotten in the catalog,
but really only /Plone/folder exists.
[maurits]

- Fixed error sorting UIDs when one of them is None. [maurits]

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- To get all brains, use ``catalog.getAllBrains`` when available.
This is available in ``Products.ZCatalog`` 2.13.30+ or 4.1+.
We fall back to ``unrestrictedSearchResults`` and otherwise just call the catalog.
Fixes `issue 16 <https://github.com/collective/collective.catalogcleanup/issues/16>`_.
[maurits]

- `Bug 22 <https://github.com/collective/collective.catalogcleanup/issues/22>`_:
Fix startup error when Archetypes is missing.
Fix other Plone 5.2 and Python 3 errors.
Should be compatible with Plone 4.3, 5.1 and 5.2 now. [maurits]