Removing unneeded from __future__ import statements.

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

ebook_converter/utils/monotonic.py

@@ -1,6 +1,3 @@
-# vim:fileencoding=utf-8
-from __future__ import absolute_import, division, print_function, unicode_literals
-
 try:
     from time import monotonic
 except ImportError: