py/objtype: Support full object model for get/set/delitem special meths.

This makes these special methods have the same calling behaviour as other
methods in a class instance (mp_convert_member_lookup() is already called
by mp_obj_class_lookup()).

tests/basics/class_staticclassmethod.py

@@ -17,9 +17,24 @@ class C:
     def __add__(self, rhs):
         print('add', rhs)
 
+    # subscript special methods wrapped in staticmethod
+    @staticmethod
+    def __getitem__(item):
+        print('static get', item)
+        return 'item'
+    @staticmethod
+    def __setitem__(item, value):
+        print('static set', item, value)
+    @staticmethod
+    def __delitem__(item):
+        print('static del', item)
+
 c = C()
 
 c.f(0)
 c.g(0)
 c - 1
 c + 2
+print(c[1])
+c[1] = 2
+del c[3]