Minor simplification and clarification

Replicates graphql/graphql-js@7f5fe4d

Author: Cito

src/graphql/execution/execute.py

@@ -1483,7 +1483,8 @@ def execute_subscription(
         root_type,
         context.operation.selection_set,
     )
-    response_name, field_nodes = next(iter(root_fields.items()))
+    first_root_field = next(iter(root_fields.items()))
+    response_name, field_nodes = first_root_field
     field_name = field_nodes[0].name.value
     field_def = schema.get_field(root_type, field_name)
 

src/graphql/validation/rules/single_field_subscriptions.py

@@ -70,8 +70,7 @@ def enter_operation_definition(
                     )
                 )
             for field_nodes in fields.values():
-                field = field_nodes[0]
-                field_name = field.name.value
+                field_name = field_nodes[0].name.value
                 if field_name.startswith("__"):
                     self.report_error(
                         GraphQLError(