you can't sub-class cStringIO.StringIO on Python 2.7

example.sh

$ python
Python 2.7.14 (default, Sep 22 2017, 00:06:07) 
[GCC 4.2.1 Compatible Apple LLVM 8.1.0 (clang-802.0.42)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import cStringIO, StringIO, io
>>> class Foo(io.StringIO): pass
... 
>>> class Bar(StringIO.StringIO): pass
... 
>>> class Baz(cStringIO.StringIO): pass
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Error when calling the metaclass bases
    cannot create 'builtin_function_or_method' instances
>>>