Django-concurrency

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* drop support Python < 3.5
* add support Django 2.2 / 3.0
* drop support Django < 1.11

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* drop official support to Django < 1.10
* add support Django 2.1
* removed deprecated api `concurrency_check`
* BACKWARD INCOMPATIBLE: version field is now mandatory in any save operation. Use ``VERSION_FIELD_REQUIRED=False`` to have the old behaviour.
* `disable_concurrency` now has `start()`, `finish()` to be called as command

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* closes :issue:`81`. Add docs and check.
* fixes :issue:`80`. (thanks Naddiseo for the useful support)
* Django 1.11 compatibility
* some minor support for Django 2.0

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* fixes bug in ConditionalVersionField that produced 'maximum recursion error' when a model had a ManyToManyField with a field to same model (self-relation)

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* just packaging

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* drop support for Python < 3.3
* add support Django>=1.10
* change license
* fixes :issue:`36`. (thanks claytondaley)
* new :setting:`IGNORE_DEFAULT` to ignore default version number