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/paginator.py

@@ -1,5 +1,4 @@
 # -*- coding: utf-8 -*-
-from __future__ import print_function, unicode_literals
 
 import functools
 import logging
@@ -7,8 +6,6 @@ import os
 from collections import namedtuple
 from math import ceil
 
-import six
-
 logger = logging.getLogger(__name__)
 PaginationRule = namedtuple(
     'PaginationRule',
@@ -131,7 +128,7 @@ class Page(object):
 
         prop_value = getattr(rule, key)
 
-        if not isinstance(prop_value, six.string_types):
+        if not isinstance(prop_value, str):
             logger.warning('%s is set to %s', key, prop_value)
             return prop_value