Django-plans

* New VAT standard rates 2017
* Remove dependency on `django.contrib.sites`.
* Feature 53: Cleaner and more explicit exception handling on `Plan.get_default_plan `.
* Feature 59: Adding code coverage to code base.
* Support customised user models by using `django.contrib.auth.get_user_model()`.
* Add Database migrations.

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* Add `AbstractOrder.return_order()`

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* Simplify the query generated by get_initial_number

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* Re-use old RecurringUserPlan in set_plan_renewal() to retain ID and history
* `autorenew_accounts` management command: better output, --providers option
* Blackify whole source

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* Prevent duplicated invoice creation when concurrent Order.complete_order() are callled
* Test in Python 3.11

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* Accidentaly same as 1.0.1