Packaging the license file

[jakirkham]

It would be good to add the license and license_files fields to setup to ensure downstream users both know the license of the software and receive the license file in their install. Also here are some more details about files that get packaged.

Note: license_file use to be used for this purpose, but has been deprecated. Now wheel and setuptools don't use it. Instead license_files (note s) is used instead.