py: Add built-in super.

tests/bytecode/mp-tests/class6.py

@@ -0,0 +1,7 @@
+class A:
+    def f(self):
+        pass
+
+class B(A):
+    def f(self):
+        super().f()

tests/bytecode/mp-tests/class7.py

@@ -0,0 +1,6 @@
+# accessing super, but not as a function call
+
+class A:
+    def f():
+        #x = super
+        print(super)