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Breaking changes:
* The type annotations now require mypy 1.0 or higher to validate correctly.
Features & fixes:
* Create values for fields of non-``list`` or ``dict`` composite types (such as ``frozenset`` or ``OrderedDict``) as
that type, instead of ``list`` or ``dict``.
* Allow overriding the field for specific composite types through the ``serializer_field_mapping`` dictionary.
* Don't set optional fields to ``rest_framework.fields.empty`` sentinel value when used in non-partial mode. This fixes
occasional leaks of the sentinel into dataclasses returned to the user, for example when a ``DataclassSerializer`` was
nested inside a regular serializer. Not setting, and later stripping, the sentinels also increases performance.
* Support dataclasses with fields that have ``init=False``.
* Support ``save()`` on serializers with ``many=True``.
* Support for fields with union types.
* Support nested serializers with ``source='*'``.
* Fix ``child_kwargs`` defined in dataclass field metadata (as opposed to ``extra_kwargs`` field on ``Meta`` subclass).