remove future (#6065)

superset/models/user_attributes.py

@@ -1,9 +1,4 @@
 # -*- coding: utf-8 -*-
-from __future__ import absolute_import
-from __future__ import division
-from __future__ import print_function
-from __future__ import unicode_literals
-
 from flask_appbuilder import Model
 from sqlalchemy import Column, ForeignKey, Integer
 from sqlalchemy.orm import relationship