Zope.app.form

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- use zope.browser.interfaces.ITerms instead of zope.app.form.browser.interfaces

- Depending on zope.schema>=3.5a1 which uses the builtin ``set`` instead of the
``sets`` module.

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- The URIDisplayWidget doesn't render an anchor for empty/None values.

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- Get rid of deprecated usage of LayerField from
zope.app.component.back35, replaced by
zope.configuration.fields.GlobalInterface.

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- Fixed restructured text in doc tests to unbreak the PyPI page.

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- Dropdown widgets display an item for the missing value even if the field is
required when no value is selected. See zope/app/form/browser/README.txt on
how to switch this off for BBB.

- Source select widgets for required fields are now required as well. They
used not to be required on the assumption that some value would be selected
by the browser, which had always been wrong except for dropdown widgets.