Otherwise, PyPI wil pick the ones for the first uploaded wheel, which means that Panda3D will not be indexed under all Python versions on PyPI.
Also add a few more classifiers.
This makes it possible to run pytest in the root directory. It also lets us store metadata such as the current version number, preventing us from having this in several different places, and allowing us to phase out parsing dtool/PandaVersion.pp.
rdb