Zope.schema

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- Implement a ``defaultFactory`` attribute for all fields. It is a callable
that can be used to compute default values. The simplest case is::

Date(defaultFactory=datetime.date.today)

If the factory needs a context to compute a sensible default value, then it
must provide ``IContextAwareDefaultFactory``, which can be used as follows::

provider(IContextAwareDefaultFactory)
def today(context):
return context.today()

Date(defaultFactory=today)

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- Rename the validation token, used in the validation of schema with Object
Field to avoid infinite recursion:
``__schema_being_validated`` became ``_v_schema_being_validated``,
a volatile attribute, to avoid persistency and therefore,
read/write conflicts.

- Don't allow "[\]^`" in DottedName.
https://bugs.launchpad.net/zope.schema/+bug/191236

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- Improve error messages when term tokens or values are duplicates.

- Fix the buildout so the tests run.

==================

- fix validation of schema with Object Field that specify Interface schema.

==================

- Prefer the standard libraries doctest module to the one from zope.testing.

==================

- Avoid maximum recursion when validating Object field that points to cycles

- Make the dependency on ``zope.i18nmessageid`` optional.