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 `AbstractRecurringUserPlan.renewal_triggered_by`
* Use `AbstractRecurringUserPlan.renewal_triggered_by` instead of `has_automatic_renewal`; `has_automatic_renewal` is not used anywhere anymore; `has_automatic_renewal=True` is automatically migrated to `renewal_triggered_by=TASK` and `has_automatic_renewal=False` to `renewal_triggered_by=USER`
* Rename `AbstractRecurringUserPlan.has_automatic_renewal` to `_has_automatic_renewal_backup_deprecated` so it can be used make your own data migration from the former `has_automatic_renewal` to `renewal_triggered_by` if the default one does not work for you
* Add `AbstractRecurringUserPlan.has_automatic_renewal` property that issues a deprecation warning and uses `renewal_triggered_by` under the hood
* Add `renewal_triggered_by` parameter to `AbstractUserPlan.set_plan_renewal`
* Deprecate `AbstractRecurringUserPlan.has_automatic_renewal`; use `AbstractRecurringUserPlan.renewal_triggered_by` instead
* Deprecate `AbstractRecurringUserPlan._has_automatic_renewal_backup_deprecated`; use `AbstractRecurringUserPlan.renewal_triggered_by` instead
* Deprecate `has_automatic_renewal` parameter of `AbstractUserPlan.set_plan_renewal`; use `renewal_triggered_by` instead
* Deprecate `None` value of `renewal_triggered_by` parameter of `AbstractUserPlan.set_plan_renewal`; use an `AbstractRecurringUserPlan.RENEWAL_TRIGGERED_BY` instead

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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