ReturnDict fails with union operator (PEP 584)

Simplest case. Under Python 3.9:

>>> from rest_framework.serializers import Serializer
>>> Serializer().data | {}

Expected output:

{}

Actual behaviour:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/luke/devel/django-rest-framework/rest_framework/utils/serializer_helpers.py", line 17, in __init__
    self.serializer = kwargs.pop('serializer')
KeyError: 'serializer'

I will provide a PR soon.

Checklist

[ ] I did not raise a discussion, as this is obviously a bug
[X] This cannot be dealt with as a third party library. (We prefer new functionality to be in the form of third party libraries where possible.)
[X] I have reduced the issue to the simplest possible case.

encode / django-rest-framework

ReturnDict fails with union operator (PEP 584) #8301

Checklist