Introduce flake8 return

Based on !2281 (merged)

I've made one commit per rule. I'm not sure I agree with:

• RET502
• RET503.

Please tell me what you think, I can easily remove them.

arkindex/documents/serializers/elements.py

@@ -667,8 +667,7 @@ class ElementSerializer(ElementTinySerializer):
 
             validated_data.update(image=image, polygon=polygon)
 
-        instance = super().update(instance, validated_data)
-        return instance
+        return super().update(instance, validated_data)
 
 
 class ElementNeighborsSerializer(serializers.ModelSerializer):