Simplify super() calls to Python 3 syntax. (#8344)

This converts calls like super(Foo, self) -> super().

Generated with:

    sed -i "" -Ee 's/super\([^\(]+\)/super()/g' **/*.py

tests/test_state.py

@@ -125,7 +125,7 @@ class StateGroupStore:
 
 class DictObj(dict):
     def __init__(self, **kwargs):
-        super(DictObj, self).__init__(kwargs)
+        super().__init__(kwargs)
         self.__dict__ = self