How to get cls information from class method?

fromkeys.py

# dict.fromkeys(...) could not set the class information for subclass correctly.
# so I want to correct it.
# Original code is here:
#    auto* fromkeys_func
#        = new BoxedFunction(FunctionMetadata::create((void*)dictFromkeys, DICT, 3, false, false), { None });
#    dict_cls->giveAttr("fromkeys", boxInstanceMethod(dict_cls, fromkeys_func, dict_cls));
# I think we should not hardcode the cls information here. And also I want to change it to BoxedBuiltinFunctionOrMethod.
# But I don't know how to get the class information if use `fromkeys` like a class method, for example:
class dictlike(dict):
    pass

a = dictlike()
b = a.fromkeys("b")
# in Pyston, get the cls information by
# Box* dictFromKeys(Box* self, ...) {
#     BoxedClass* cls = self->cls;

a = dictlike.fromkeys("a")
# how to get the cls information in this situation in Pyston?
# Neither `cls = self->cls;` nor `cls = static_cast<BoxedClass*>self` will work.