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

tests/test_ext_coverage.py

@@ -37,7 +37,7 @@ def test_build(app, status, warning):
     undoc_py, undoc_c = pickle.loads((app.outdir / 'undoc.pickle').bytes())
     assert len(undoc_c) == 1
     # the key is the full path to the header file, which isn't testable
-    assert list(undoc_c.values())[0] == set([('function', 'Py_SphinxTest')])
+    assert list(undoc_c.values())[0] == {('function', 'Py_SphinxTest')}
 
     assert 'autodoc_target' in undoc_py
     assert 'funcs' in undoc_py['autodoc_target']