Django-choices

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* Fixed inability to set custom order to 0 (50), thanks to kavdev for the
patch and robinramael for the report
* Added API to get the attribute name from a value (48), thanks to jaseemabid
for the report

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* Dropped support for old Python/Django versions.
* Added support for ``NullBooleanField`` -- thanks to ashwch
* Added retention of choices order in ``DjangoChoices.values`` -- thanks to merwok

.. warning::
Dropped support for Python versions < 2.7 and 3.3, and Django < 1.8. If you
need explicit support for these versions, you should stick to version 1.4.4.

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* Bugfix for better IPython support (125d523e1c94e4edb344e3bb3ea1eab6f7d073ed)

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Fixed a bug in the validator error message output - thanks to Sobolev Nikita

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This is a small release that fixes some ugliness. In Django 1.7 and up, the
validator can now be used as documented in the readme. Before this version, the
error::

ValueError: Cannot serialize: <bound method DjangoChoicesMeta.validator of <class 'choices.models.MyModel.Choices'>>

would be raised, and a workaround was to define a validator function calling the
choices' validator, requiring everything to be defined in the module scope.

This is now fixed by moving to a class based validator which is deconstructible.

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* Added support for upcoming Django 1.9, by preferring stlib SortedDict over
Django's OrderedDict
* Added pypy to the build matrix
* Added coverage to the Travis set-up