Django-oscar-docdata

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* Added missing parameter when raising a `DocDataStatusError` exception in the sandbox
* Wrap the order update in `order_status_changed` in a transaction
* Wrap `_store_report_lines` in a transaction and make sure `payment_id` is a string
* Simplified `_store_report_lines` by using `get_or_create`

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* remove `DOCDATA_ORDER_ID_START` from sandbox
* renamed remaining createErrors / cancelErrors / statusErrors to be compliant with Docdata API version 1.3

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* createError has been renamed to createErrors in Docdata API version 1.3

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* Upgraded to Docdata 1.3 API
* Removed the untested and probably unused `start` method from the `DocdataClient` class
* Removed the corresponding start_payment method from the `Interface` class

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* Added Python 3 and Django classifiers and wheel package
* Dropped Django < 1.11 and added support for Django 1.11, 2.0 and 2.1
* Fixed management commands to work with Django>=1.11
* Cleanup and simplified sandbox application
* Removed unused `django-polymorphic` dependency and cleanup up depending model

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* Added Python 3 support.