Narrow setup 'Operating System' classfiers to known-good.

We know for a fact pants does not work out of the box on Windows
as reported here:
  https://groups.google.com/d/topic/pants-devel/N6CKjY5j2ps/discussion

For now, restrict the claimed OS support to those we know pants
works with / we have CI for.

Also add appropriate 'Topic' classifiers.

https://rbcommons.com/s/twitter/r/1075/