Django-strategy-field

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* sort names in combo box

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* `import_error` now expects new argument with original exception

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* add `import_error` attribute
* BACKWARD INCONPATIBLE: do not raise exception if ImportError
* drop support django < 1.10
* add support for DjangoRestFramework `contrib.drf`
* improved test suite

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* fixes django 2.0 compatibility

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* add support django 2.0
* drop support django <= 1.8
* registry can be callable
* do not register class used as Registry base class

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* add 'display_attribute' to customize how registered classes should be displayed
* fixes display in Admin site change list
* Registry now accept string as fully qualified class name as argument or None to by-pass class checking
* Registry.register() now accept string as fully qualified class name as argument to allow a kind of "lazy" registration
* new `contrib` package with some helper class