Modernize code base to Python 3+ syntax

Replaces syntax that was relevant in earlier Python versions but that
now has modernized equivalents.

pelican/writers.py

@@ -1,5 +1,3 @@
-# -*- coding: utf-8 -*-
-
 import logging
 import os
 from urllib.parse import urljoin
@@ -16,7 +14,7 @@ from pelican.utils import (get_relative_path, is_selected_for_writing,
 logger = logging.getLogger(__name__)
 
 
-class Writer(object):
+class Writer:
 
     def __init__(self, output_path, settings=None):
         self.output_path = output_path