Removing unneeded from __future__ import statements.

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

ebook_converter/ebooks/conversion/plugins/rtf_input.py

@@ -1,13 +1,12 @@
-from __future__ import with_statement, unicode_literals
-__license__   = 'GPL v3'
-__copyright__ = '2008, Kovid Goyal <kovid at kovidgoyal.net>'
-
 import os, glob, re, textwrap
 import pkg_resources
 
 from ebook_converter.customize.conversion import InputFormatPlugin, OptionRecommendation
 from ebook_converter.polyglot.builtins import iteritems, filter, getcwd, as_bytes
 
+__license__   = 'GPL v3'
+__copyright__ = '2008, Kovid Goyal <kovid at kovidgoyal.net>'
+
 border_style_map = {
         'single' : 'solid',
         'double-thickness-border' : 'double',