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* new action "Find Duplicates"
* allows customization of exportable columns (`get_exportable_columns(request)`)
* allows customization od ExportForm (`get_export_form(request, export_type)`)
* Add Admin logging support (LogEntry)
* add Celery support
* MassUpdateForm sort fields by label
* BACKWARD INCOMPATIBLE: removed `modeladmin` from signals arguments
* BACKWARD INCOMPATIBLE: signals `request` argument is now optional to support async execution
* BACKWARD INCOMPATIBLE: Drop support for Django 2.x
* BACKWARD INCOMPATIBLE: Removes internal api SelectOptionsAttribute