Initial pass of removing Python 2 support

This commit removes Six as a dependency for Pelican, replacing the
relevant aliases with the proper Python 3 imports. It also removes
references to Python 2 logic that did not require Six.

pelican/tests/support.py

@@ -1,5 +1,4 @@
 # -*- coding: utf-8 -*-
-from __future__ import print_function, unicode_literals
 
 import locale
 import logging
@@ -10,12 +9,11 @@ import sys
 import unittest
 from contextlib import contextmanager
 from functools import wraps
+from io import StringIO
 from logging.handlers import BufferingHandler
 from shutil import rmtree
 from tempfile import mkdtemp
 
-from six import StringIO
-
 from pelican.contents import Article
 from pelican.readers import default_metadata
 from pelican.settings import DEFAULT_CONFIG