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/templates/pelicanconf.py.jinja2

@@ -1,6 +1,5 @@
 #!/usr/bin/env python
 # -*- coding: utf-8 -*- #
-from __future__ import unicode_literals
 
 AUTHOR = {{author}}
 SITENAME = {{sitename}}

pelican/tools/templates/publishconf.py.jinja2

@@ -1,6 +1,5 @@
 #!/usr/bin/env python
 # -*- coding: utf-8 -*- #
-from __future__ import unicode_literals
 
 # This file is only used if you use `make publish` or
 # explicitly specify it as your config file.