py/runtime: Fix self arg passed to classmethod when accessed via super.

Thanks to @AJMansfield for the original test case.

Signed-off-by: Damien George <damien@micropython.org>

tests/basics/subclass_classmethod.py

@@ -35,3 +35,34 @@ class B(A):
 B.bar() # class calling classmethod
 B().bar() # instance calling classmethod
 B().baz() # instance calling normal method
+
+# super inside a classmethod
+# ensure the argument of the super method that is called is the child type
+
+
+class C:
+    @classmethod
+    def f(cls):
+        print("C.f", cls.__name__)  # cls should be D
+
+    @classmethod
+    def g(cls):
+        print("C.g", cls.__name__)  # cls should be D
+
+
+class D(C):
+    @classmethod
+    def f(cls):
+        print("D.f", cls.__name__)
+        super().f()
+
+    @classmethod
+    def g(cls):
+        print("D.g", cls.__name__)
+        super(D, cls).g()
+
+
+D.f()
+D.g()
+D().f()
+D().g()