Z3c-preference

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- Add support for Python 3.5, 3.8 and 3.9.

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- Update tests to `zope.testbrowser >= 5`.

- Add support for Python 3.6 and 3.7.

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- Sorting preferences in ``CategoryEditForm`` by their `id` to stabialize
sort order.

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- Descriptions of preference groups are now rendered in the ``group-header``
slot of the form above the error messages for the group.

- Fixed description of version `0.3`, it actually added descriptions for
preference categories, not preference groups.

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- Descriptions of preference categories are now rendered in the
``extra-info`` slot of the form.

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- Added form for preference categories, see `Editing preference group trees`_.