Removing unneeded from __future__ import statements.

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

ebook_converter/polyglot/builtins.py

@@ -1,12 +1,7 @@
-#!/usr/bin/env python2
-# vim:fileencoding=utf-8
-# License: GPL v3 Copyright: 2018, Kovid Goyal <kovid at kovidgoyal.net>
-
-from __future__ import absolute_import, division, print_function, unicode_literals
-
 import os
 import sys
 
+
 is_py3 = sys.version_info.major >= 3
 native_string_type = str
 iterkeys = iter