Now that #582 landed, the github-served artifact is small

-rw-rw-r-- 1 alexeagle alexeagle 2.8M Dec 28 14:52 beed9a257cc493ebf5aa7b8e6bbe577fa637e8f6.zip
-rw-rw-r-- 1 alexeagle alexeagle 338K Dec 28 14:51 main.zip

so there's no need for rules_python to distribute a release artifact, we can just let GitHub serve a source archive.
This lets us use a much simpler release pattern from https://github.com/bazel-contrib/rules-template where maintainers just push a tag and the rest is automated.