io.py contains the following to declare ABCs for some of its classes:

class IOBase(_io._IOBase, metaclass=abc.ABCMeta):
    pass

(and similarly for RawIOBase, BufferedIOBase, TextIOBase).

_io._IOBase has an extensive docstring, but IOBase doesn't. (Python doesn't inherit parent-class docstrings, on the theory that the subclass may change things that make the docstring invalid.)

I propose the attached patch.