Python-3-only clean ups discovered by pyupgrade

https://github.com/asottile/pyupgrade

> A tool to automatically upgrade syntax for newer versions of the
> language.

- Drop u str prefix
- Drop base object inheritance
- Drop args to super()
- Use set literals
- Use dict comprehension
- Use set comprehension

doc/conf.py

@@ -48,7 +48,7 @@ epub_fix_images = False
 epub_max_image_width = 0
 epub_show_urls = 'inline'
 epub_use_index = False
-epub_guide = (('toc', 'contents.xhtml', u'Table of Contents'),)
+epub_guide = (('toc', 'contents.xhtml', 'Table of Contents'),)
 epub_description = 'Sphinx documentation generator system manual'
 
 latex_documents = [('contents', 'sphinx.tex', 'Sphinx Documentation',