Sub-classes of `str` appear to just operate as aliases of `str`

```py
>>>>> class String(str):
.....     def foo(self):
.....         return "foo"
..... 
>>>>> String()
'' 
>>>>> String().foo()
Traceback (most recent call last):
  File <unknown>, line 0, in <module>
AttributeError: RefCell { value: [PyObj class "str"] } object has no attribute foo
>>>>> type(String())
<class 'str'> 
```