Fix type of UnionField to accept not only Field but also Serializer

[ROCKTAKEY]

Hello, thanks for your maintenance.

I fixed the constructor type of UnionField to accept not only Field but also subclass of Field, such as Serializer.

As described in Common issues and solutions / variance in mypy document, Dict[T, U] type does not accept subclass of U, while Mapping does. So I replace Dict with Mapping.

Example

The code below causes error by mypy. After applying this PR, it does NOT causes any error.

from dataclasses import dataclass

from rest_framework.fields import IntegerField

from rest_framework_dataclasses.fields import UnionField
from rest_framework_dataclasses.serializers import DataclassSerializer


@dataclass
class Example:
    x: int


class ExmapleSerializer(DataclassSerializer[Example]):
    class Meta:
        dataclass = Example


child_fields = {
    Example: ExmapleSerializer,
    int: IntegerField,
}

UnionField(child_fields=child_fields)

main.py:24: error: Argument "child_fields" to "UnionField" has incompatible type "dict[type, type]"; expected "dict[type, Field[Any, Any, Any, Any] | type[Field[Any, Any, Any, Any]]]"  [arg-type]
main.py:24: note: "Dict" is invariant -- see https://mypy.readthedocs.io/en/stable/common_issues.html#variance
main.py:24: note: Consider using "Mapping" instead, which is covariant in the value type
Found 1 error in 1 file (checked 1 source file)

[oxan]

Thanks!