io classes incomplete in Python 2

[ddfisher]

IOBase and BufferedIOBase have no methods in Python 2. They are, however, implemented in _io -- we should consider copying them over.

[JukkaL]

If they are the same class, we could just expose one of the definitions in the other module. For example, we can add from _io import IOBase as IOBase into the io stub.

[JelleZijlstra]

I think #1395 fixed this.