[PATCH 3 of 3 v2] demandimport: reject contextlib._GeneratorContextManager on Py < 3.2

[timeless]

please hold off on v2. I'd like to try a v3 that handles
distutils.msvc9compiler -- it turns out that processfromlist might
really be too late and that I may have to really split out the
iteration of fromlist into two places.