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

utils/jssplitter_generator.py

@@ -79,7 +79,7 @@ function splitQuery(query) {
 }
 ''' % (fold(singles, ','), fold(ranges, '],'))
 
-js_test_src = u'''
+js_test_src = '''
 // This is regression test for https://github.com/sphinx-doc/sphinx/issues/3150
 // generated by compat_regexp_generator.py
 // it needs node.js for testing