A few setup.py improvements, removing future lib (#6219)

superset/utils/dashboard_import_export.py

@@ -1,9 +1,4 @@
 # 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 json
 import logging
 import time