UPGRADE FROM 5.2 to 5.3
=======================
Form
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* Changed `$forms` parameter type of the `DataMapperInterface::mapDataToForms()` method from `iterable` to `\Traversable`.
* Changed `$forms` parameter type of the `DataMapperInterface::mapFormsToData()` method from `iterable` to `\Traversable`.
* Deprecated passing an array as the second argument of the `DataMapper::mapDataToForms()` method, pass `\Traversable` instead.
* Deprecated passing an array as the first argument of the `DataMapper::mapFormsToData()` method, pass `\Traversable` instead.
* Deprecated passing an array as the second argument of the `CheckboxListMapper::mapDataToForms()` method, pass `\Traversable` instead.
* Deprecated passing an array as the first argument of the `CheckboxListMapper::mapFormsToData()` method, pass `\Traversable` instead.
* Deprecated passing an array as the second argument of the `RadioListMapper::mapDataToForms()` method, pass `\Traversable` instead.
* Deprecated passing an array as the first argument of the `RadioListMapper::mapFormsToData()` method, pass `\Traversable` instead.
HttpKernel
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* Marked the class `Symfony\Component\HttpKernel\EventListener\DebugHandlersListener` as internal
Security
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* Deprecated voters that do not return a valid decision when calling the `vote` method.
