Removing unneeded from __future__ import statements.

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

ebook_converter/utils/serialize.py

@@ -1,9 +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
-
 from ebook_converter.polyglot.builtins import unicode_type
 from ebook_converter.constants import ispy3