Removing unneeded from __future__ import statements.

Since we are on Python 3.6 and up, we don't need those anymore.

ebook_converter/ebooks/html_entities.py

@@ -1,8 +1,3 @@
-#!/usr/bin/env python2
-# vim:fileencoding=utf-8
-# License: GPLv3 Copyright: 2017, Kovid Goyal <kovid at kovidgoyal.net>
-from __future__ import absolute_import, division, print_function, unicode_literals
-
 html5_entities = {
 # ENTITY_DATA {{{
     'AElig': 'Æ',