Automatically set version support classifiers when wheel file specifies such support

[brettcannon]

E.g., https://pypi.python.org/pypi/reportlab has wheel files with Python 3 support but they didn't bother to set their trove classifiers stating as such. Since Warehouse already has this support information thanks to the wheel files it would be handy if it also set the appropriate trove classifiers for the developers as well when they forget to set them manually.

[dstufft]

I'm on the fence on this. On one hand I agree it would be useful, but I also worry that it's getting close to "guessing" in the face of ambiguity and also removing editorial control from project authors over their own project. It'll wait until after Warehouse becomes PyPI in either case (unless someone else makes a PR) but it might be worthwhile poking distutils-sig to see if anyone is opposed to it.