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.

Testing Done:
$ pants.dev setup_py src/python/pants:pants-packaged
*** Running pants in dev mode from /home/jsirois/dev/3rdparty/jsirois-pants/src/python/pants/bin/pants_exe.py ***
WARNING!  /home/jsirois/dev/3rdparty/jsirois-pants/dist/pantsbuild.pants.m5cE9B/src/pants/docs/conf.py is source but does not belong to a package!
WARNING!  /home/jsirois/dev/3rdparty/jsirois-pants/dist/pantsbuild.pants.m5cE9B/src/pants/docs/pagerank.py is source but does not belong to a package!
Running packager against /home/jsirois/dev/3rdparty/jsirois-pants/dist/pantsbuild.pants-0.0.24
Writing /home/jsirois/dev/3rdparty/jsirois-pants/dist/pantsbuild.pants-0.0.24.tar.gz

$ tar -xzf dist/pantsbuild.pants-0.0.24.tar.gz -C dist/
$ tail dist/pantsbuild.pants-0.0.24/PKG-INFO
        * Initial published version of ``pantsbuild.pants``.

Platform: UNKNOWN
Classifier: Intended Audience :: Developers
Classifier: License :: OSI Approved :: Apache Software License
Classifier: Operating System :: MacOS :: MacOS X
Classifier: Operating System :: POSIX :: Linux
Classifier: Programming Language :: Python
Classifier: Topic :: Software Development :: Build Tools

$ pants.dev setup_py src/python/pants:test_infra
*** Running pants in dev mode from /home/jsirois/dev/3rdparty/jsirois-pants/src/python/pants/bin/pants_exe.py ***
Running packager against /home/jsirois/dev/3rdparty/jsirois-pants/dist/pantsbuild.pants.testinfra-0.0.24
Writing /home/jsirois/dev/3rdparty/jsirois-pants/dist/pantsbuild.pants.testinfra-0.0.24.tar.gz
$ tar -xzf dist/pantsbuild.pants.testinfra-0.0.24.tar.gz -C dist/
$ tail dist/pantsbuild.pants.testinfra-0.0.24/PKG-INFO

Platform: UNKNOWN
Classifier: Intended Audience :: Developers
Classifier: License :: OSI Approved :: Apache Software License
Classifier: Operating System :: MacOS :: MacOS X
Classifier: Operating System :: POSIX :: Linux
Classifier: Programming Language :: Python
Classifier: Topic :: Software Development :: Build Tools
Classifier: Topic :: Software Development :: Testing

Bugs closed: 593, 609

Reviewed at https://rbcommons.com/s/twitter/r/1075/
Sync of https://github.com/pantsbuild/pants/commit/c60fd9cc08294ec70d11cb5da120433a2c5e2468