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/tools/pelican_import.py

@@ -1,6 +1,5 @@
 #!/usr/bin/env python
 # -*- coding: utf-8 -*-
-from __future__ import print_function, unicode_literals
 
 import argparse
 import logging
@@ -11,10 +10,9 @@ import sys
 import time
 from codecs import open
 from collections import defaultdict
-
-from six.moves.urllib.error import URLError
-from six.moves.urllib.parse import quote, urlparse, urlsplit, urlunsplit
-from six.moves.urllib.request import urlretrieve
+from urllib.error import URLError
+from urllib.parse import quote, urlparse, urlsplit, urlunsplit
+from urllib.request import urlretrieve
 
 # because logging.setLoggerClass has to be called before logging.getLogger
 from pelican.log import init
@@ -24,7 +22,7 @@ from pelican.utils import SafeDatetime, slugify
 try:
     from html import unescape  # py3.5+
 except ImportError:
-    from six.moves.html_parser import HTMLParser
+    from html.parser import HTMLParser
     unescape = HTMLParser().unescape
 
 logger = logging.getLogger(__name__)
@@ -396,19 +394,8 @@ def posterous2fields(api_token, email, password):
     """Imports posterous posts"""
     import base64
     from datetime import timedelta
-    try:
-        # py3k import
-        import json
-    except ImportError:
-        # py2 import
-        import simplejson as json
-
-    try:
-        # py3k import
-        import urllib.request as urllib_request
-    except ImportError:
-        # py2 import
-        import urllib2 as urllib_request
+    import json
+    import urllib.request as urllib_request
 
     def get_posterous_posts(api_token, email, password, page=1):
         base64string = base64.encodestring(
@@ -451,19 +438,8 @@ def posterous2fields(api_token, email, password):
 
 def tumblr2fields(api_key, blogname):
     """ Imports Tumblr posts (API v2)"""
-    try:
-        # py3k import
-        import json
-    except ImportError:
-        # py2 import
-        import simplejson as json
-
-    try:
-        # py3k import
-        import urllib.request as urllib_request
-    except ImportError:
-        # py2 import
-        import urllib2 as urllib_request
+    import json
+    import urllib.request as urllib_request
 
     def get_tumblr_posts(api_key, blogname, offset=0):
         url = ("http://api.tumblr.com/v2/blog/%s.tumblr.com/"