init: Remove __future__ statements or py2

The __future__ statements we used were:
 - absolute_import - Mandatory in 3.0
 - with_statement - Mandatory in 2.6
 - print_function - Mandatory in 3.0

As we only support python >= 3.6 - this can be removed now

pypandoc/__init__.py

@@ -1,5 +1,4 @@
 # -*- coding: utf-8 -*-
-from __future__ import absolute_import, print_function, with_statement
 from typing import Iterable
 from typing import Union
 from typing import Generator

pypandoc/py3compat.py

@@ -1,7 +1,5 @@
 # -*- coding: utf-8 -*-
 
-from __future__ import with_statement
-
 import locale
 import sys