Plone.app.z3cform

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Bug fixes:

- a11y: added role attribute for portalMessage
[nzambello]

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New features:

- Add support for rendering <optgroup> elements from
zope.schema.interfaces.ITreeVocabulary hierarchical terms.
[rpatterson]

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Bug fixes:

- Fix deprecation warning
(https://github.com/plone/Products.CMFPlone/issues/2605) [ale-rt]

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Bug fixes:

- Prepare for Python 2 / 3 compatibility
[pbauer, MatthewWilkes, ale-rt]

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Bug fixes:

- Cleanup code analysis problems.
[jensens]

- Fix SingleCheckBoxBoolWidget description to render structure
[allusa]

- Prepare for Python 2 / 3 compatibility
[pbauer, MatthewWilkes, ale-rt]

- Render mimetype selection box correctly for a ``RichTextWidget`` which also
brings back the TinyMCE.
[sallner]

- Allow RelatedItems widget to be used in subforms
[tomgross]

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Bug fixes:

- Fix test failures from https://github.com/plone/plone.app.widgets/pull/177
[thet]

- Prepare for Python 2 / 3 compatibility
[pbauer]