Update install.py

Drop support for python < 3.6

Author: demotu

src/jupyter_contrib_nbextensions/install.py

@@ -1,10 +1,6 @@
 # -*- coding: utf-8 -*-
 """API to install/remove all jupyter_contrib_nbextensions."""
 
-from __future__ import (
-    absolute_import, division, print_function, unicode_literals,
-)
-
 import errno
 import os