Z3c-form

3.3.0 besides cherry-picks.)
Fixes: https://github.com/zopefoundation/z3c.form/issues/41

- Host documentation at https://z3cform.readthedocs.io

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- Add support for Python 3.6.

- Drop support for Python 3.3.

- Avoid duplicated IDs when using a non-required field with
``z3c.formwidget.query.widget.QuerySourceRadioWidget``.
[pgrunewald]

------------------

- Drop support for Python 2.6.

- Support Python 3.5 officially.

- Fix TypeError: object of type 'generator' has no ``len()``.
Happens with z3c.formwidget.query. [maurits]

- Turned ``items`` into a property again on all widgets.
For the select widget it was a method since 2.9.0.
For the radio and checkbox widgets it was a method since 3.2.10.
For orderedselect and multi it was always a property.
Fixes https://github.com/zopefoundation/z3c.form/issues/44
[maurits]

- Fix handling of missing terms in collections. (See version 2.9 describing
this feature.)

- Fix `orderedselect_input.js` resource to be usable on browser layers which do
not extend ``zope.publisher.interfaces.browser.IDefaultBrowserLayer``.

------------------

- *MAJOR* overhaul of ObjectWidget:

* low level unittests passed, but high level was not tops
basic rule is that widgets want RAW values and all conversion
must be done in ``ObjectConverter``

* ``ObjectSubForm`` and ``SubformAdapter`` is removed,
it was causing more problems than good

* added high level integration tests

- Removed ``z3c.coverage`` from ``test`` extra. [gforcada, maurits]

-------------------

- RadioWidget items are better determined when they are needed [agroszer]

- CheckBoxWidget items are better determined when they are needed [agroszer]

- Bugfix: The ``ChoiceTerms`` adapter blindly assumed that the passed in field
is unbound, which is not necessarily the case in interesting ObjectWidget
scenarios. Not it checks for a non-None field context first. [srichter]

------------------

- Correctly handled ``noValueToken`` in RadioWidget. This avoids a
``LookupError: --NOVALUE--``. [gaudenz,ale-rt]

- Added ``json`` method for forms and ``json_data`` method for
widgets. [mmilkin]

- Change javascript for updating ordered select widget hidden structure so it
works again on IE11 and doesn't send back an empty list that deletes all
selections on save. Fixes https://github.com/zopefoundation/z3c.form/issues/23
[fredvd]

- Started on Dutch translations.
[maurits]