remove future imports (#6050)

superset/data/__init__.py

@@ -1,10 +1,5 @@
 """Loads datasets, dashboards and slices in a new superset instance"""
 # pylint: disable=C,R,W
-from __future__ import absolute_import
-from __future__ import division
-from __future__ import print_function
-from __future__ import unicode_literals
-
 import datetime
 import gzip
 import json