Your class inherits from object
and is thus not a classic class. It is a new-style class. (Any class that inherits from a new-style class is also a new-style class.)
Note that super
can't be used with classic classes anyway. The "depth first left to right" rule you cite is just the attribute lookup rule for the order base classes are searched when you try to access an attribute that is not directly defined on the instance. Classic classes don't provide an MRO as such --- there isn't any way to access "the superclass value" of an attribute without explicitly referencing a particular superclass.