[PATCH 3 of 3 v3] demandimport: reject contextlib._GeneratorContextManager on Py < 3.2

[timeless]

Siddharth Agarwal wrote:
> As a general thing I don't think we should be spending time trying to fix
> this implementation of demandimport on Python 3 at all.

This isn't really about python3, it's just about foreign code that
expects a certain object to look a certain way, but because of
demandimport it gets a demandimport object stub instead of an
exception -- in python3 it would have at least gotten a stub for a
real object. -- I don't know if the code works in py3 but that's a
different adventure, and I'm happy to let a python3 demandimport
solution worry about it.