Add registration_schema to DraftRegistrationDetailSerializer

Recent DRF upgrade fixed a day-one bug which affects MRO. Have to
explicity define registration_schema to prevent this property from
inheriting from the wwrong parent.

Co-authored-by: John Tordoff <johnetordoff@gmail.com>

api/draft_registrations/serializers.py

@@ -16,6 +16,7 @@
     NodeContributorsSerializer,
     NodeContributorsCreateSerializer,
     NodeContributorDetailSerializer,
+    RegistrationSchemaRelationshipField,
 )
 from api.taxonomies.serializers import TaxonomizableSerializerMixin
 from osf.exceptions import DraftRegistrationStateError
@@ -143,6 +144,13 @@ class DraftRegistrationDetailSerializer(DraftRegistrationSerializer, DraftRegist
     registration_supplement, node, cannot be changed after draft has been created.
     """
 
+    registration_schema = RegistrationSchemaRelationshipField(
+        related_view='schemas:registration-schema-detail',
+        related_view_kwargs={'schema_id': '<registration_schema._id>'},
+        required=False,
+        read_only=False,
+    )
+
     links = LinksField({
         'self': 'get_self_url',
     })