PEP 584 dict union operator incompatible with Django REST Framework ReturnDict

[zach-waggoner]

Python 3.9 introduced PEP 584, which added the dict union operator (|). Unfortunately, this does not appear to work with ReturnDict. For example, this works:

s = Serializer()
d = {**{}, **s.data}

But this throws KeyError: 'serializer':

s = Serializer()
d = {} | s.data

If feasible, it would be helpful to support PEP 584, especially because pyupgrade transforms the former into the latter, likely because ReturnDict is a subclass of dict.